To reduce the test error,the orthogonal double beam spectrophotometer was designed. The orthogonal double beam spectrophotometer uses one photoelectric conversion system which includes one photodiode,one filtration and amplification,one AD conversion and adds a producer of orthogonal signal which is a rotatable disk and non-opaque somewhere. In each cycle the reference signal and sample signal get through the producer of orthogonal signal and project to the photoelectric conversion system. In this way,the signals collected by the photoelectric conversion system in each cycle are composite and consist of the reference signal,sample signal and dark current signal,each of which will be separated by the master control system of FPGA as each code and restored for the following calculation and analysis. Since the error caused by the differences of photoelectric transducer and collecting time is avoided theoretically,the measurement accuracy of double beam spectrophotometer can be improved through taking one photoelectric conversion system and adding a producer of orthogonal signal. The experimental results show that the orthogonal double beam spectrophotometer has advantages in stability and accuracy.