Optical parametric oscillation (OPO) or optical parametric amplification (OPA) systems offer significant potential for generating high-energy, narrow-pulse laser output, finding applications across various domains. To achieve efficient amplification in cascade with amplifiers, precise frequency domain matching between OPO/OPA systems and amplifiers is imperative. Therefore, preliminary simulation and computation of the spectral characteristics of OPO/OPA systems are of paramount importance in the early stages of experimentation. Currently, the calculation of OPO/OPA output spectrum characteristics is limited to estimating the influence of pump light characteristics on output spectrum linewidth, a consideration that remains incomplete. This paper presents a comprehensive computational approach that takes into account the impact of four crucial factors on the output spectrum characteristics: pump light linewidth, pump light divergence angle, the walk-off effect, and the absorption loss of the crystal. This method finely characterizes the features of the output spectrum. The results obtained through the proposed approach align well with experimental results, underscoring its effectiveness. This study aims to obtain the output spectrum characteristics of BaGa4Se7 OPO/OPA through calculations to provide theoretical guidance for subsequent cascade amplification experiments.