Numerical simulations of rarefied gas flows in pumping channels of a helical-type drag pump (HTD) are carried out by using the direct simulation Monte Carlo (DSMC) method. Performance curves obtained by the DSMC method in the slip flow regime are compared with those obtained by the Navier-Stokes equations with slip boundary conditions. Satisfactory agreement between the two results is achieved. Also, the previously developed diffusion model [J. Vac. Sci. Technol. A 13, 2592 (1995)] is modified to predict the performance of a HTDP. Experimental data are presented in the pressure range of 0.02-4 Torr. The numerical results obtained by the modified diffusion model are in good agreement with the experimental data over the entire pressure range from the molecular to the slip flow.