A temperature detecting device for temperature detecting using the temperature sensitive feature of environmentally sensitive hydrogel is provided, including a metal housing, an environmentally sensitive hydrogel, a slider, a built-in slide rail, an organic slat, a graduation line and an organic housing. The device is in contact with the measured object, and the environmentally sensitive hydrogel exchanges heat with a measured object through the metal housing and makes varying degrees of volume changes, to promote the slider slip along the built-in slide rail and eventually to the equilibrium state. When the slider is finally static, the temperature value is read from the graduation line. The temperature detecting device can realize the measurement and monitoring of the temperature, and is fast in response speed and not easy to be damaged. The structure is simple, the cost is low, the security is stable and the device size is not limited.