In order to improve the temperature control precision of the bellows-type J-T cooler, the needle design is optimized. The tip of the needle is grooved to form double throttled holes with the main throttled hole on the valve body of the cooler. The working medium enters the main throttled hole for throttling and cooling after the start. The hole is completely blocked by the needle after self-regulating of the cooler, and the steady small flow is ejected from the groove.The reasonable size of the groove is determined. The test results show that the groove on the tip of the needle can effectively improve the temperature control precision.