An Image acquisition device for a CMOS digital image sensor typically uses a high-performance processor or CPLD/FPGA. When the image is gray and its resolution is not high, MCU can be applied. There are many acquisition methods employed. This paper describes a binary image inquiry method based on the microcontroller Serial Peripheral Interface (SPI). Using the SPI interface can decrease requirements for MCU performance and reduce MCU's frequency. This method is easy to implement gray threshold through hardware and use less system resources.