A high-speed back illuminating InAlAs avalanche photodiode(APD) was developed to meet the requirements of single-channel 25Gbit/s long-distance transmission. The APD adopts vertical back-illumination separate-absorption-grading-charging-multiplication (SAGCM) structure. Triple-layer mesa was formed by etching process, and the electric field was limited to the center of the maximum mesa multiplier layer, effectively reducing the mesa edge breakdown. The chip adopts reverse welding structure with integrated microlens on the back to improve optical coupling aperture. The responsivity of the developed APD chip is 0.84A/W (λ=1310nm) at the gain of M=1. When M=10, 3dB bandwidth reaches 19GHz. The gain-bandwidth product is 180GHz. The best sensitivity at 5×10-5 bit error rate is -22.3dBm, which can support 100GBASE-ER4 communication standard.