Abstract: A new pseudo natural sampling algorithm is proposed in this paper for correcting the harmonic distortion produced by switching signal modulation in Uniform sampled Pulse Width Modulation (UPWM) digital class D power amplifiers. This algorithm combines the 3th-order and 1st-order Lagrange interpolation methods and adopts a position judging method for pseudo natural sampling point to obtain an expected harmonic distortion correction effect and computation complexity. A switching signal modulator test system based on FPGA is built to test the proposed algorithm and other kindred algorithms. The test results show that the proposed algorithm basically eliminates the harmonic distortion with modest hardware consumption and is better than other kindred algorithms.