Abstract: One LN2 cryopump with 900 mm internal diameter is described in this paper,which is applied to environmental simulation equipment,and pumping speed for N₂ is over 30 000 L/s.The cryopump can achieve the vacuum according to cryogenic condensation and cryogenic adsorption process by using circulating liquid nitrogen,and working together with GM cryocooler to supply cooling capacity commonly.This paper mostly discusses the operating principle,the structure design,the heat load calculation and the specification testing.It can provide the references of design ideas and computing methods for big caliber cryopump.