Optical pumped quasi-monochromatic and tunable terahertz (THz) wave generation and detection system is investigated theoretically. Calculations are performed on an organic nonlinear medium: 4′-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST), due to the superior properties (large dynamic range and extremely wide tunability). The behavior of pulses during the nonlinear interaction is characterized. Previous experimental results are well explained. Our theoretical model provides an approach to improve the performance of the coherent THz system from two aspects. For the source part, an optimal crystal thickness that maximizes the THz output is obtained. For the detector part, a linear conversion relation (used for calibration) and the parameter dependence of responsivity are provided. It was predicted that a dynamic range of ten orders could be achieved with the DAST-based THz system. The presented model can also be applied to guide the design of THz systems with other nonlinear mediums in collinear phase-matching geometry.