High-speed CMOS image sensors (CISs) have the advantages of high integration, high frame rate, low power consumption, and radiation difficulty. It is widely used in scientific experiments. When used in image measurement and diagnosis, CIS usually works at a synchronized mode triggered by an external signal. This mode, called the transient imaging mode, is quite different from the continuous imaging mode in which the sensor outputs images frame-by-frame at a specific frame rate. In this paper, the performance of a high-speed CIS that has 5T pixels and a global shutter is analysed in transient imaging mode, and the key performance of the sensor is tested using an EMVA1288 compatable device and compared with continuous imaging mode. The results show that in transient imaging mode, CIS has a larger dark current and a lower signal-to-noise ratio and dynamic range. However, the temporal readout noise and photo response nonuniformity are better. The test results could be used in scientific imaging system design and performance optimization.