Most of the reports on Innoslab master-oscillator power-amplifier (MOPA) amplifiers are about experimental results or models that do not adequately take into account all the factors that affect the performance of Innoslab amplifiers. A modified Frantz–Nodvik (F-N) equation and a simple finite-element slice model based on graphical methods for the Innoslab MOPA laser system are theoretically developed for the first time. The pump absorption saturation effect, the pump transverse spatial distribution, the spatial overlap between the seed and pump, the influence of the tilted optical path and repetition rate, and the overlap effect of the non-collinear beam in the slow axis are fully included. Based on the above model, the performance of a partially end-pumped MOPA system is studied and designed. At the repetition rate of 1 kHz and pulse width of 10 ns, an output pulse energy of 64 mJ is obtained. The consistency of the experimental results and the numerical simulation indicates that this model is a powerful tool to optimize and design a high-power Innoslab MOPA system. The method has been applied to the mass production of lasers with good economic benefits.