Abstract: Theoretic analysis reveals that, ALTS(adaptive local time step) method offered by Laux, although greatly reducing the execution CPU time needed for the flow to reach to a steady state and thus leading to high simulation efficiency of DSMC(direct simulation of Monte Carlo) method codes, has some drawbacks in complicating molecules’ moving and collision calculations and in requiring additional RAM. Modifications were made to time intervals at which molecular moving and collision are computed, and to the collision sampling method. These modifications simplified molecules’ moving and collision computation, and also avoid the consumption of unnecessary RAM. The modified ALTS method was adopted in the DSMC code. Simulations of rarefied gas flow over a cylinder were performed with and without this modified ALTS method, and the result shows that these modifications result in a significant reduction in the execution CPU time, but introduce little difference on the simulation resolution and the flow structure.