Abstract: Since the systematic nonlinear error is the primary factor that influences the photometric precision of spectrophotometer and the common methods such as chemistry standard method and neutral density filter method for calibrating the photometric scale of a spectrophotometer can not meet the needs for calibrating the photometric scale of high precision spectrophotometers which require the uncertainty of photometric scale to be less than 0.001 (but the common methods is confined to 0.005), the doubleaperture method is brought forward in this paper. The measurement principle of the doubleaperture method for calibrating photometric scale of spectrophotometers and the way for calibrating Lambda 950 Spectrophotometer are introduced in details. The nonlinear parameters of Lambda 950 Spectrophotometer were obtained with the method. The nonlinear error was derived for the instruments working in 250nm～2500nm. The accuracy of photometric scale, preceding 0.0005, was acquired after calibration with the parameters.