A high-precision and digital true root mean square measurement method based on Field Programmable Gate Array(FPGA) is presented. Firstly, FPGA is employed to design Finite Impulse Response(FIR) filter to filter the AD sampled data. Furthermore, the improved RMS formula is adopted to calculate the true RMS of the signal. The mean value of the true RMS value of eight consecutive cycles is taken as the final measurement result. By designing the algorithms of serial extraction and division operations, the use of FPGA resources is reduced. The actual test of the prototype shows that the relative error between the measurement results and the true value of the signal is less than 0.5%. The solution has high measurement accuracy, good consistency, and less resources, which has certain reference value for the design of the true RMS digital measurement chip.