Double junction photodiode includes one deep and one shallow pn junctions, and the optimal current ratio I2/I1 between deep and shallow junctions increases with the raise of the incident wavelength. In this paper, the mathematical modeling and Matlab simulation of the photocurrent for deep and shallow junction diodes are realized based on 0.5 μm CMOS technology. The current ratio of deep and shallow junction for double junction photodiode is converted to the voltage by the integrated signal processing circuits. The simulation results show that the output of the signal processing circuits has a good linear relationship with the natural logarithm of ln(I2/I1). The monolithic integrated CMOS wavelength detection chip can be used for the detection and analysis of unknown wavelength.