We numerically investigate the effects of the time delay between prepulse and main pulse on the lasing characteristics of the Ni-like Mo soft x-ray lasers (XRL) at 18.9 nm driven in a grazing incidence pumping (GRIP) geometry by using a modified 1D hydrodynamic code MED103 coupled with an atomic physics data package and a 2D Ray-tracing code. Typical XRL experiments with lasing action in a broad temporal window for the delay are simulated and analyzed. Investigations show a well-performed preplasma together with the proper main pulse is the key to achieve lasing over a broad range of time delay.