The beam loading effect is a considerable issue for a high average current beam.We apply a novel method without utilizing the specious beam current value to rigorously analyze the beam loading effect in S-band linacs.By tracking every macro-particles state and using the energy conservation law,power dissipated from the beam is calculated in each cell.In the new particle-based algorithm,the concept of equivalent beam current is proposed.Its value is not constant and it has a small variation through the accelerating structure.Also,we introduce the iteration algorithm as comparison and we find that the two algorithms coincide with each other verv well.