The spaceborne photon counting laser ranging radar system has significant advantages such as high repetition rate and high precision, but it also faces the problem of large amount of original data and high proportion of noise data. In order to adapt to the transmission capacity of the satellite data channel, it is necessary to compress the original data volume and ensure the recall rate of signal photons, so it is necessary to develop a hardware based real-time denoising algorithm. This article proposes a fast denoising algorithm that combines coarse and fine processing. Firstly, based on the laser emission pulse width, system noise rate, target characteristics, and local density information of received photon events, coarse denoising is performed to remove some noisy photons; then, using histogram statistics, fine denoising is performed on the retained photon events to determine the signal photon interval and the final signal photon and its time information. The algorithm is verified by Monte Carlo simulation and ICESat-2 measured data. The test results show that the recall ratio, precision ratio and harmonic average of the algorithm are more than 94%, 93% and 94% respectively, and the operational efficiency is improved by 10%. The algorithm can achieve fast and real-time denoising of photon events, providing a theoretical basis for real-time denoising processing of onboard hardware.