In view of the problem that the traditional calculation of surface nuclear magnetic resonance (SNMR) excitation field seldom considers various three-dimensional subsurface electric properties and different forms of launcher, the authors carried out surface NMR three-dimensional finite element modeling which premeditates the impact of a conductive medium. First, based on the three-dimensional finite element method, the authors realized the calculation of surface NMR excitation field under the condition of inhomogeneous medium. An approximate function was adopted to replace field source, so that it could be applied to different cases of transmitter loop. Then, the vertical component of excitation field was obtained through rotation matrix. Finally, the NMR response under different conditions was calculated. Several typical models were designed to analyze the effect of cap rock resistivity, formation resistivity, inhomogeneous medium and transmitter coil posture on SNMR signal. Numerical simulation shows that nuclear magnetic resonance is not sensitive to the change of the high cap rock resistivity. Low formation resistivity has great effect on SNMR response. The larger the inhomogeneous medium, the greater the SNMR response. For a certain size of the local inhomogeneous body, the difference between the SNMR response curves under the condition of low resistance is obvious. The coil posture should be considered in the field because it considerably impacts the NMR signal, and the coil orientation should be considered because the influence of the coil orientation on the magnetic signal is greater than electrical parameters.