Abstract: The marine controlled-source electromagnetic method (CSEM) is a new technology to detect subsea hydrocarbon reservoirs by electromagnetic fields of low frequency. Its prospecting capability is studied by numerical modeling in this paper. Based on 1D marine model, the electromagnetic responses on seafloor is calculated for different water depth environments and hydrocarbon reservoirs with various buried depth, thickness and resistivities. The biggest relative anomaly and corresponding absolute anomaly (i.e. amplitude difference) of electric fields for each buried depth, thickness and resistivity are plotted and the influence of these parameters on the electromagnetic anomaly is analyzed. The simulation results show that the marine controlled-source electromagnetic method has great performance in subsea hydrocarbon reservoirs exploration. Even the reservoirs of small thickness, small resistivity difference or large buried depth in shallow sea environment can lead to considerable electromagnetic anomalies. When other factors are same, the vertical impedance of oil layer decides the biggest electric anomaly. To achieve the optimal performance of marine CSEM, appropriate working frequency and receiver offset are necessary. The study results will provide useful references for the geophysical scheme design of future subsea hydrocarbon exploration.