This paper presents a novel bunch current measurement system based on an ultrafast photodetector and a high-speed digitizer at Hefei light source II (HLS II). We use a metal-semiconductor-metal photodetector to measure the emitted optical synchrotron radiation intensity directly, representing the bunch current intensity. To achieve bunch-by-bunch resolution, the sampling rate of the system is nearly 225 GS/s, which is achieved via a dedicated equivalent sampling algorithm. The detailed description of the experimental setup and the equivalent sampling algorithm are presented. According to preliminary tests of the daily operation mode and single-bunch mode, the measured root-mean-square of the beam current is 1%, which shows that the new system satisfies the requirements for high-precision bunch current measurements. In addition, experimental results of the “HLS” Morse-code fill pattern mode demonstrate that this system could also be a convenient and robust tool for beam top-up modes in the future.