The principle and methods of linearity measurement are discussed.An intelligent measurement system is designed for linearity of a photoelectric detector.The system is based on the method of two lights addition for irradiance and the principle of light blended in integral global.The photoelectric characteristic of supper brightness LED is measured.The result proves that the supper brightness LED is an excellent light source.The course of measurement is auto-controlled by the computer.The theory and structure of the system are discussed.The measured nonlinearity is discussed and analyzed.A silicon photoelectric detector is measured for many times.The repeated precision of measurement is prior to ±0.5 %.Measurement results prove the design of the system has high automation and precision.This system can be used as instrument of linear measurement of the photoelectric detectors.