Abstract: Based on theories of the heat and mass transfer inside the cold trap, a model to compute impurities captured by forced circulation cold trap was built. In the process of modeling, an algorithm to calculate the heat transfer was developed, applying to the non-isothermal cold trap which has a regenerator inside; meanwhile, a program was compiled. The calculated results show that the total mass captured by the cold trap and the capture efficiency increase with the captured time, but the amplifications gradually decline; impurities in the sodium-potassium alloy mainly precipitate on the wire mesh, and the mass of impurities captured by the wire mesh is much larger than the total mass captured by three walls; the shorter lattice length and the larger diameter of the wire mesh are beneficial to capturing impurities while the lattice length has more effects on the captured time than the diameter of the wire mesh.