High-speed light source is realized for decoy-state quantum key distribution (QKD) at telecom wavelength of 1.55 μm. By implementing two different electrical pulses together and triggering with 100 MHz pseudorandom number to drive the laser diode, the signal-state and the decoy-state pulses are prepared with identical pulse duration of 25 ps and similar spectral characteristics, avoiding the eavesdropper's attack by temporal and spectral analysis. The intensity fluctuation of the light source is quantified to satisfy the practical decoy-state QKD with random intensity error. The characteristics of the light source are analyzed with a high-speed single-photon detector.