A method based on tunable semiconductor laser absorption spectroscopy (TDLAS) is used to study the problem of trace water measurement error due to pressure change during the drying process of electric core, and realize the contactless, high-precision and real-time detection of water vapor content. Firstly, according to the Lambert-Beer law, it is found that the changes in water vapor absorption spectra, optical range, and pressure will affect the measurement efficiency. The water vapor absorption spectra and the effective optical range of the measurement at 105 ℃ were investigated by the method of HITRAN simulation, and the results show that the water molecules are absorbed best and there is no interference of the background gases when the laser operates in the band of 7 181.17 cm^-1. Absorption coefficients of 10^-3 can be achieved at an optical range of 12 m. The change in pressure will lead to the change of the second harmonic peak and affect the water vapor measurement. By studying the absorption of water vapor under different pressures, a positive correlation between pressure and absorption coefficient is found, and a normalization method is proposed to solve the measurement error caused by the change of pressure by taking advantage of this law. The experimental tests found that the peak fluctuation of the second harmonic compensated by the pressure was reduced from 350 to 6.2 mV. The results of five experiments showed that the water content ratio to the second harmonic's peak voltage was 0.24, and the measurement error after calibration was less than 1.2%. These results are significant in improving the detection accuracy of water vapor content in the battery drying process.