In order to study effects of trimmed double suction impeller on pressure pulsation in a double volute case, the first stage of a multistage volute pump was selected as a study object, steady and unsteady fluid flow simulations in the pump were performed based on the N-S equations, RNG k-ε turbulence model when the first stage double suction impeller was trimmed under different conditions, such as six trimmed quantities and four trimming patterns. Pressure fluctuations in time and frequency domains at monitoring points near the tongue and at the volute outlet point were obtained. The results show that impeller trimmed quantity has a greater influence on the pressure change near the tongue of double volute case, and the pressure variation amplitude is decreased firstly and then increases with increasing trimmed quantity, especially the pressure fluctuation is the smallest when the quantity is 0.98. Some trimming patterns can reduce the pressure fluctuation near the tongue in some degree, the pressure fluctuations under V cut and forward skew cut are smaller than those under the straight cut and backward skew cut, and the change in performance with backward skew cut is little smaller than the straight cut. At the volute outlet point, the pressure fluctuation is decreased with increasing trimmed quantity, but is less affected by the trimming patterns.