Based on the current application requirements for wideband response photodetectors, we designed a novel silicon avalanche photodetector (Si APD) offers high response in a broad spectral range of 250-1100nm and is capable to achieve efficient detection of three bands of ultraviolet, visible and near-infrared light without the need of splicing. The enhancement of ultraviolet band and infrared band of silicon were separately analyzed. This was followed by simulation on the device structure designs using different methods such as back incidence, to improve short wavelength absorption while maintaining a high infrared absorption. The Si APD shows a peak wavelength at around 940nm and a high photoresponse at 250nm and 1100nm which exceed 15% of the peak responsivity. This type of device is suitable for multispectral applications and future high-precision detection.