The invention discloses an oil and gas detection method and system based on electromagnetic survey. The method comprises the following steps: acquiring interface layering information of an experimental measuring line and resistivity data of a deep well log; calculating to obtain weighted mean resistivity; building a layered resistivity two-dimensional model; setting experimental testing frequency; calculating electromagnetic response to obtain original amplitude data and original phase data of the layered resistivity two-dimensional model; performing noise removal and filter to obtain the noise-free amplitude data and noise-free phase data; carrying out differential treatment to obtain differential amplitude data or differential phase data; selecting and calculating the average value corresponding to a high-frequency section in the differential amplitude data or differential phase data; returning to zero to obtain abnormal lithology amplitude data or abnormal lithology phase data; calculating to obtain the residual amplitude electromagnetic data or the residual phase electromagnetic data; drawing a cross-section diagram of an isoanomalic contour line of pseudo-depth domain according to the residual amplitude electromagnetic data or the residual phase electromagnetic data; then generating the oil-gas detection result according to the cross-section diagram of the isoanomalic contour line of pseudo-depth domain.