Organic photodetector (OPD) with high sensitivity has broadband response from visible light to near infrared (NIR) and excellent overall device performance. It plays a very important role in various applications including high-quality biological imaging. In this paper, Silvaco TCAD used to simulate a broadband organic photodetector whose active layer was made of a mixture of a broad band gap polymer PBDTTT-C-T as a donor and a newly developed fused octacylic small molecule FOIC as a receptor. The simulation results show that the dark current density, the external quantum efficiency and the lowest detectable light intensity ability of the device reach high standard and fit well with the experiment data. In view of this, it is believed that the parameters used in the simulation process have good reliability and value, which can provide useful reference for the simulation and research of the same type of photodetector.