Microstrip antenna has attracted extensive attention in the field of seafloor detection and underwater radar due to its stable structure and strong beam directivity. In this letter, the total-field scattered-field source finite-difference time-domain method with a simplified calculation method is considered to analyze the electromagnetic wave propagation generated by an underwater microstrip antenna (UMA). We analyze the field distributions of UMA, reduced-scale antenna and the two-element array composed of two UMAs. Meanwhile, the effect of element spacing on the radiation field distribution generated by the array is specifically studied. In addition, the scattering field of a metallic ellipsoidal target is analyzed. The simulations show that seawater can be used as the medium layer of the UMA in order to generate energy distribution in a near region similar to that of the conventional microstrip antenna. Furthermore, the element spacing for the underwater array should be adjusted based on the practical application.