Based on the coupling characteristics of waveguide and microcavity in two-dimensional photonic crystals, an all-optical 4×2 encoder is proposed. The encoder realizes a high-contrast fast encoding function by introducing a microcavity into the Y-shaped waveguide. The performance of the proposed 4×2 encoder is studied by using plane wave method (PWM) and finite-difference time-domain (FDTD) method. The results show that the proposed encoder has a contrast ratio up to 24.55 dB, a response time not exceeding 1.11 ps, a data transmission rate not less than 0.90 Tbit/s, and a size of about 147.45 μm² at a working wavelength of 1.55 μm. The proposed encoder has simple structure, high contrast, fast response speed, and easy integration, which is expected to play an important role in the fields of optical communications and integrated optical circuits.