Optical power density monitoring and temperature monitoring play an important role in the industrial production and day-to-day life. In this study, an all fiber-optic Mach-Zehnder sensor with PbSe quantum dots as the sensing material is fabricated on the basis of thermo-optical characteristic of PbSe quantum dots, and the sensor is tested under different optical power densities and temperatures. The results show that the power density sensitivity of the sensor to a light with a wavelength of 473 nm is 1.455 nm·(mW-1·mm2) and the temperature sensitivity is 0.67 nm·℃-1. This study not only realizes a high-sensitivity temperature sensor, but also lays the foundation for the thermal-optical devices made by PbSe quantum dots.