The measurement of optical power is one of the most important and commonly used measurement techniques in the field of optical research, especially in the field of optical communication, laser mapping and so on. At the same time, the measurement of optical power with better than pW level sensitivity and large dynamic range is a very key technology for system testing and calibration. However, the measurement sensitivity of commercial optical power meters usually can only reach nW level. In this paper, an optical power measuring device with limit sensitivity up to the sparse photon level is developed. The measuring system bases on the silicon avalanche photodiode (Si APD), simultaneously employs the combination of diffuser and electronic diaphragm to achieve precise intensity control of the measured signal light. The accurate optical power measurement in the range of 20fW～300W is achieved, and the response wavelength range of power measurement can cover 530～860nm. The measurement error is proved less than 3.7%, and the relative standard deviation of repeatability is measured less than 1.9%, which can well meet the requirements of the weak optical signal power measurement. This optical power measurement method with both of high sensitivity and large dynamic range is expected to be used in many fields which are facing the difficulty of metering the weak signal in sparse photon level.