A new approach for model conversion of a state-delay system is proposed in this paper. The derivation and the main results are all in the state-space formation such that the further application and design of a signal system can be more convenient. To proceed this work in continuous-to-discrete model conversion, the digitally sampling period is larger than the delay time is considered, and the discrete-time model is especially derived from the continuous-time system so that it has a wider range for sampling period in real system implementation. Moreover, a reasonably larger sampling period in the digital model can decrease the computation time in the processor system. In the discrete-to-continuous model conversion, a simple method is also proposed to determine the unknown state-delay time from the available discrete-time model, and thus the generated continuous-time model is established through some existing relationship, and the delay time also can be evaluated for the originally continuous model of physical systems.