A novel Doppler frequency shift (DFS) measurement system based on a cascaded modulator topology is presented. It is capable to measure both the object speed and moving direction on an electrical spectrum analyzer with the use of a fixed low-frequency reference signal. It has a simple single-laser and single-photodetector structure, a robust performance and a wide operating frequency range, which is only limited by the optical modulator bandwidth. Experimental results are presented that demonstrate DFS ranging from -100 kHz to +100 kHz with an error of less than ±0.1 Hz at different microwave signal frequencies. Over 35 dB suppression in unwanted frequency components present at the system output is also demonstrated.