The invention relates to a low-
pressure diffusion process applied to polycrystalline 
black silicon solar cells, which comprises the following steps: (1) allowing a 
silicon wafer to entering a tube; (2) performing constant temperature 
processing; (3) performing low temperature oxidation, generating a 
thin oxide layer on a surface of the 
silicon wafer to make the subsequent 
phosphorus source deposition more uniform; (4) performing 
low temperature deposition, uniformly depositing a 
phosphorus source on the surface of the 
silicon wafer; (5) performing high temperature propulsion, allowing the 
phosphorus source to diffuse into the silicon wafer, wherein during high temperature propulsion, the temperature is 820-850 DEG C, the 
nitrogen flow rate is 1000-3000sccm, the dry 
oxygen is 0-1000sccm, the furnace pressure is 50-150mbar, and the time is 10-20 minutes; (6) performing secondary 
diffusion, wherein during the secondary 
diffusion, the temperature is 800-850 DEG C, the 
nitrogen flow rate is1000-3000sccm, the source 
nitrogen is 0-400sccm, the dry 
oxygen is 0-1000sccm, the furnace pressure is 50-150mbar, the time is 2-10 minutes; (7) performing cooling; (8) performing nitrogen filling, allowing the pressure inside the 
pipe to reach the 
atmospheric pressure so that a furnace door is opened; and (9) performing discharging. The present invention improves the uniformity of the square resistance after 
diffusion of polycrystalline 
black silicon.