This Letter presents a real-time coherent receiver using digital signal processing (DSP)-assisted automatic frequency control (AFC) to compensate for the Doppler frequency shift (DFS). DFS compensation range of ±8 GHz and the frequency shifting rate of 33 MHz/s are demonstrated in an FPGA-based 2.5 Gbaud QPSK coherent optical system. The experimental results indicate that the scheme achieves a sensitivity of -47 dBm at a bit error rate (BER) of 2E-4. The power penalty induced by the DFS compensation is less than 1 dB.