The invention provides an evaluation method for the under-compaction and fluid expansion overpressure of a stratum. According to the method, a stratum resistivity-stratum density relation layout is constructed by means of overpressure and normal pressure stratum data selection and stratum resistivity and stratum density extraction; and therefore, the under-compaction overpressure and fluid expansion overpressure of the stratum can be effectively and simply evaluated, and at the same time, the ratio of the contribution of the under-compaction overpressure and fluid expansion overpressure to theoverpressure of the stratum can be calculated.