Abstract: An InGaAsmetalsemiconductormetal (MSM) photodetector was designed with interdigitated planar structure. Theoretical models were used to plot the variations of the dark current, the photocurrent, the capacity, and the cutoff frequency of the photodetector as a function of bias voltage and the interelectrode distance. The dark current of the detector was restricted in orders of pA by adding InAlAs Schottky barrier. The simulation results show that the detector has obvious light response. Under certain circumstances, the working frequency of the detector can reach 1.5 THz. Moreover,the samples of detector were fabricated, the dark current and photo response of the samples were tested. The test results are coincident well with the simulation results.