In order to further explore the new detection technology of terahertz based on cesium Rydberg atom, we studied the radiation lifetime after atomic transition and the sensitivity of system noise limitation under different transition modes (S_1/2→P_3/2、D_5/2→P_1/2、D_5/2→P_3/2) using simulation under four-level Rydberg atomic model. The simulation results show that the atomic radiation lifetime after transition increases with the increase of its energy level principal quantum number. Among the three transition modes of the model, the atomic radiation lifetime of S_1/2→P_3/2 is shorter than the other two transition modes. For the shot noise limited sensitivity, the sensitivity value of D_5/2 →P_1/2 transition mode is the smallest, that is, the detection sensitivity of the system will be the highest in this transition mode. This conclusion provides a reference for the Redburg atomic terahertz detection technology and lays a foundation for weak signal detection in the field of biology and materials.