A thermal imaging apparatus for measuring a temperature of a target in a monitored area comprises a thermal imager, an optical image capturing device and a computing processing device. The thermal imager is configured to capture a thermal image of the monitored area. The optical image capturing device is configured to capture optical images of the monitored area. The computing processing device is configured to determine one of the optical images as a determined optical image synchronizing with the thermal image according to positions of blocks corresponding to the target in the thermal image and the optical images, perform calculation according to the thermal image and the determined optical image to obtain a measured distance between the target and the thermal imaging apparatus, and perform calibration according to the measured distance and the thermal image to obtain a calibrated temperature value of the target.