NOVELTY - An iridium (Ir) and iridium-platinum (Ir-Pt) alloy nanometer catalyst is prepared by using compressed hydrogen reduction method and taking carbon black modified by potassium borohydride (KBH4) as carrier; and reducing Ir or Ir and Pt precursor compound and depositing Ir or Ir-Pt alloy nanoparticle on surface of modified carbon black carrier and obtaining desired catalyst, where carbon black is Vulcan XC-72 (RTM: carbon black).    USE - Method for preparation of iridium (Ir) and iridium-platinum (Ir-Pt) alloy nanometer catalyst (claimed) used in chemical process, fuel cell, aerospace engineering, pollutant emission control and biological engineering.    ADVANTAGE - The catalyst has high dispersity and high and controllable capacity. The method is simple and has low cost and low requirement for equipment.    DETAILED DESCRIPTION - An iridium (Ir) and iridium-platinum (Ir-Pt) alloy nanometer catalyst is prepared by using compressed hydrogen reduction method and taking carbon black modified by potassium borohydride (KBH4) as carrier; and reducing Ir or Ir and Pt precursor compound and depositing Ir or Ir-Pt alloy nanoparticle on surface of modified carbon black carrier and obtaining desired catalyst, where carbon black is Vulcan XC-72 (RTM: carbon black). The method includes:    (A) adding carbon black carrier in secondary distilled water, where mass ratio of carbon black to secondary distilled water is 1: 500;    (B) carrying out ultrasound treatment for 20 minutes and adding KBH4 to form solution whose concentration is 1 mol/L;    (C) reacting while stirring for 8 hours, filtering and cleaning with secondary water (50 mL), carrying out vacuum drying at 60 degrees C for 2 hours and obtaining modified processed carbon black;    (D) adding polyethylene pyrrolidone (1 wt.%) into iridium precursor water solution whose Ir content is 0.01 wt.% as protecting agent, where mass ratio of Ir to polyethylene pyrrolidone is 1:3;    (E) stirring for 20 minutes and adding obtained modified processed carbon black, where mass ratio of Ir to carbon black is 5.26-66.67:100;    (F) carrying out ultrasound treatment for 30 minutes and removing dispersed reaction solution in high pressure reaction kettle and introducing hydrogen until 4 MPa;    (G) heating to 150 degrees C and reacting 7 hours, cooling to room temperature, filtering and cleaning with secondary distilled water until no chlorine can be detected;    (H) vacuum drying for 2 hours at 60 degrees C and obtaining high dispersity carbon-carried Ir nanometer catalyst whose capacity is 5-40%;    (I) adding polyvinyl pyrrolidone (1 wt.%) as protective agent into mixing water solution of iridium precursor whose Ir content is 0.01 wt.% and platinum precursor whose Pt content is 0.01 wt.%, where mass ratio of Ir to Pt is 1:0.25-4 and mass ratio of Ir to polyvinyl pyrrolidone is 1:3;    (J) stirring for 0.25 hours and adding obtained modified processed carbon black, where mass ratio of Ir/Pt alloy to carbon black is 5.26-66.67:100;    (K) carrying out ultrasound treatment for 30 minutes and removing dispersed reaction solution in high pressure reaction kettle and introducing hydrogen until 4 MPa;    (L) heating to 150 degrees C, reacting for 7 hours, cooling to room temperature, filtering and cleaning with secondary distilled water until no chlorine can be detected; and    (M) vacuum drying for 2 hours at 60 degrees C and obtaining high dispersity carbon-carried Ir-Pt nanometer catalyst whose capacity is 5-40%.