We report a temperature measurement method based on fixed wavelength transmission intensity of long-period fiber grating. Relative to wavelength or edge filtering, this method has obvious advantages in measurement accuracy and cost. The relationship between the transmission intensity and temperature at a fixed wavelength is simulated based on the theory of long-period fiber grating coupling mode. The results show that the transmission intensity of the fixed wavelength is linearly related to the temperature with the appropriate parameters. In the experiment, the transmission spectrum shows two resonant peaks at 1485 and 1538 nm. Two fixed wavelengths at 1480.4 and 1549.7 nm were selected, and the results show a linear relationship between transmission intensity and temperature. The temperature sensitivity of the transmission intensity at 1480.4 nm is about 0.0544  dB  /    °  C, which means the value of I_{1480.4  nm}  /  I₀ changes by 1.3% for every 1°C. To eliminate the influence of light source and enhance the measurement sensitivity, the ratio of the transmission intensity of two fixed wavelengths was selected to carry out the temperature measurement. The temperature sensitivity of the ratio of I_{1480.4  nm}  /  I_{1549.7  nm} was 0.0729  dB  /    °  C, that is, the light intensity ratio changed by 1.7% when the temperature changed by 1°C. A more reasonable design of long-period fiber grating and a more appropriate selection of fixed wavelength should be able to obtain higher temperature sensitivity.