NOVELTY - Gas diffusion layer comprises a carbon black layer (2) having a micropore diameter of 30-100 mu m and/or a carbon fiber layer (1) having a micropore diameter of 50-150 mu m.    USE - The gas diffusion layer is useful for fuel cell, electrolytic cell and gas diffusion electrode in electrochemical ultracapacitor.    ADVANTAGE - The gas diffusion layer has simple, reasonable and compact structure, high gas and water permeability, high conductivity, high stability of catalyst layer and high utilization ratio of catalyst.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for preparing the gas diffusion layer, comprising (i) mixing (in wt.%) a carbon fiber material (5-30), a dispersant (5-30) and deionized water to form a homogeneous slurry, then adding polytetrafluoroethylene (PTFE) emulsion and stirring for 0.5-2 hours, where the amount of PTFE emulsion is 10-50 wt.% of the weight of the carbon fiber material, and adding a viscosity adjustor to adjust the viscosity of the slurry to 1000-30000 cps to obtain a carbon fiber slurry, (ii) mixing carbon black powder (10-20) or a mixture of carbon black powder and carbon fiber (5-30) with deionized water and a dispersant to obtain a homogeneous slurry, then adding a PTFE emulsion into the slurry and stirring for 0.5-2 hours, where the amount of the PTFE emulsion is 10-50 wt.% of the weight of the carbon black powder or the mixture of carbon black powder and carbon fiber, and adding a viscosity adjustor to adjust the viscosity of the slurry to 1000-30000 cps to obtain a carbon black slurry, (iii) immersing a diffusion layer base body in PTFE emulsion for 0.5-15 minutes, taking out the base body and airing, drying it in a drying oven at 50-150 degrees C and sintering for 20-60 minutes at 300-390 degrees C to obtain a diffusion layer base body containing PTFE, and (iv) coating the diffusion layer base body containing PTFE with the carbon fiber slurry and the carbon black slurry respectively by means of screen printing, coating or spray coating, and sintering the obtained base body for 20-60 minutes at 300-390 degrees C.