In recent years, with the growth of modular multilevel converter based high voltage direct current (MMC-HVDC) transmission systems, the power loss of the main grid increases gradually. The efficient and accurate loss calculation and analysis of MMC-HVDC systems is the key step to study the loss composition and loss reduction measures. An effective method is proposed to calculate the MMC-HVDC power loss based on finite sampling. The method considers the nonlinear characteristics of pulse waveform and device impedance, and it can calculate the power loss during a long time period quickly by changing the input data, effectively resolve the challenging issue of long time period loss calculation with high accuracy. Finally, the accuracy of the algorithm is verified by comparing with the measured power loss of the actual project.