NOVELTY - A solid alkali catalyst comprises metal nitrate and alkali or alkali metal carbonate, where it is prepared by high temperature roasting after solid phase reacting, and has special process comprising placing metal nitrate and alkali or alkali metal carbonate in mortar with mass ratio of 1-12:10, rubbing for 1-2 hours at room temperature, roasting for 5-10 hours at 300-1000 degrees C. The metal nitrate is magnesium nitrate, aluminum nitrate, zinc nitrate, or strontium nitrate. The alkali or alkali metal carbonate is sodium hydroxide, potassium hydroxide, sodium carbonate, or potassium carbonate.    USE - The solid alkali catalyst is useful for preparing biodiesel (claimed).    ADVANTAGE - The invention has high yield, cheap cost, less catalyst dosage, mild reaction condition, short reaction time, reusable catalyst, no environmental pollution and, low material demand.    DETAILED DESCRIPTION - A solid alkali catalyst comprises metal nitrate and alkali or alkali metal carbonate, where it is prepared by high temperature roasting after solid phase reacting for metal nitrate and alkali or alkali metal carbonate, and has special process comprising placing metal nitrate and alkali or alkali metal carbonate into mortar with mass ratio of 1-12:10, rubbing for 1-2 hours at room temperature, roasting for 5-10 hours in muffle furnace at 300-1000 degrees C. The metal nitrate is magnesium nitrate, aluminum nitrate, zinc nitrate, calcium nitrate, or strontium nitrate. The alkali or alkali metal carbonate is sodium hydroxide, potassium hydroxide, sodium carbonate, or potassium carbonate. An INDEPENDENT CLAIM is a method of preparing biodiesel by ester changing comprising adding oil, low alcohol, solid alkali catalyst into three-neck container with electromagnetism stirrer, thermometer and reflowing condenser to heat, reflow and stir for 1-3 hours, controlling temperature at 50-110 degrees C, filtering reacted material, filtering out catalyst and filtrate, separating filtrate to separate out lower layer of glycerine, adding sodium bicarbonate (NaHCO3) solution with 5-10 wt.% into upper layer of mixture, where its dosage is 2-6 times of that of upper layer of mixture, washing the mixture to be neutral, eliminating unreacted free fatty acid, using separating hopper to separate out upper layer of organic phase, using anhydrous magnesium sulfate (MgSO4) to dry organic phase, filtering it to remove drying agent, reducing pressure to distill, and obtaining light yellow transparent biodiesel, where the mass ratio of added oil, low alcohol and solid alkali catalyst is 1:0.1-0.5:0.04-0.1, the oil is plant oil, animal oil or waste oil, pit oil in producing process, and the low alcohol is methanol, ethanol, propanol, or butanol.