The method of estimating matched field power solves the problem that the traditional method of estimating power under the assumption of spherical or cylindrical wave is greatly affected by the shallow water waveguide. However, the accuracy of the matched field power estimation is seriously affected by the environmental uncertainty. In this work, the environmental sensitivity in source power estimation is defined, and the loss of the source power estimation performance is quantified when the environment is uncertain. The relation between the loss of the source power estimation performance and the estimation deviation of the acoustic transfer function is derived by using the modulus deviation and the angle between the estimated acoustic transfer function and the ideal value. On this basis, we study the mechanism of the influence of environmental uncertainty on source power estimation, and the influences of water depth, sound speed of water and sound speed of sedimentary layer on the estimation of source power. The results show that the influences of different environmental parameters on the modulus are small, and the decline of the power estimation performance is mainly due to the fact that the angle varies greatly. The change of water depth has the most significant influence on the power estimation results, and the change of sound speed of sedimentary layer exerts little influence on the power estimation results. Beside the water depth, the change of sound speed of water has a greater influence on the power estimation results at high frequencies.