The invention relates to a soil dynamic characteristic test technique for geotechnical engineering and especially relates to a simple instrument and a method for test of an internal stress of a soil mass under cyclic loading. A large quantity of researches and reports record that the roadbed soil under the cyclic loading is suffered from dynamic stress accumulation, but the academics does not pay attention to the phenomenon. The simple instrument can be mainly used for measuring the response conditions of the internal stress in the soil on factors such as different cyclic loading magnitudes, loading waveforms, loading frequencies and soil compaction rate, and acquiring the parameters, such as settlement curve, dynamic elastic modulus, total deformation modulus, static elastic modulus, of the soil mass under the cyclic loading effect. The method provided by the invention belongs to an innovative technology which is simple in laboratory test and convenient in soil dynamic characteristic acquisition; with the help of a lever pressure meter, the instrument can realize 10-times load output and can save instrument consumable materials and power sources; a computer system can be utilized to realize the automatic control on the instrument control and the data collection.