A power-saving optimization operation method and switching point determining method for a water pump unit. In the parallel water pump units, k water pumps converters form a sub-pump unit A. The water output Q1 of a first water pump in the sub-pump unit A, the input power P1 of the frequency converter corresponding to Q1 and the operating frequency f1 of the frequency converter corresponding to Q1 are recorded, where QA=Q1, PA=P1. The QA-PA curve of an operating water pump serves as the working curve w1, where QA=mQ1 and PA=mP1, and k≥m≥2. The working curve wm of m operating water pumps operating at the same frequency is obtained, where f1=f2= . . . =fm. The intersection point of the working curve wm-1 and the working curve wm is the optimal switching point between m-1 operating water pumps and m operating water pumps under the constant pressure Hs.