Linear measurement device was set up based on the linear relationship between pump power and photons in spontaneous parametric down conversion process. The nonlinearity errors mechanism, linear condition and correction model of photon counting detector were analyzed. For the sake of realizing the linear measurement of the photon counting, the pump power was adjusted to change incident photon rate, and the experimental measurements were corrected. The results show that the measurement precision can be improved by measurement and correction. The maximum linear range rate of the incident photon could be expanded from 105/s to 107/s result from lower non-linear errors.