Abstract: To investigate input transients and longitudinal beam instabilities in high-energy electron accelerator storage rings, the beam diagnostics team at the Shanghai Synchrotron Radiation Facility (SSRF) developed a bunch-by-bunch bunch length measurement system based on oscilloscope random sampling. This system utilizes a high-speed oscilloscope to capture raw beam position monitor (BPM) signals, which are subsequently processed by data analysis scripts for bunch length calculations. However, the computational efficiency and real-time performance of this system were limited. The process is cumbersome and computationally inefficient. In the case of multiple bunches, calculating a set of data can take up to two hours, which greatly inconveniences real-time monitoring of the machine status. To optimize computational efficiency and real-time performance, a bunch-by-bunch bunch length measurement system was developed based on phase-locked sampling with a processor. This system directly acquires raw beam signals through the processor and performs bunch length calculations in real time. It can significantly streamline the data processing flow and reduce computation time. Nevertheless, the processor’s analog-to-digital converter (ADC) acquisition board faced the challenge of low sampling rates. Given that the processor is equipped with eight sampling channels, a one-to-eight equal-power-division delay signal conditioning front-end which equally spaced delays to the eight-way power-divided signals was designed. This method increased the system sampling rate eightfold through interleaved sampling. Additionally, by calibrating the consistency of each channel in the measurement system and the system’s transmission impedance, the system is able to accurately reconstruct the original BPM signal waveforms. For Gaussian-distributed beams, the system achieves a bunch-by-bunch, turn-by-turn bunch length measurement accuracy of 0.17 ps, with a dynamic range spanning thousands of turns. Furthermore, this measurement system has been successfully deployed at the Hefei Light Source (HLS) and used to observe beam instabilities in the storage ring. This system can improve the computational efficiency of bunch length measurements and enable real-time observation of bunch length variations in the storage ring. It holds potential for future deployment in other storage rings.