Terahertz (THz) transient has become an effective method to study the optical and electronic spin characteristics of the rare earth orthoferrites RFeO₃. High-throughput grown crystal sample is sliced at different locations, then the continuously tunable rare earth elements co-doped single crystal Sm_xPr_1–xFeO₃ is studied with antiferromagnetic spin mode (qAFM) and crystal field transitions of rare earth ions under zero magnetic fields. Using THz time-domain spectroscopy, the qAFM resonance frequencies of Sm_0.2Pr_0.8FeO₃ and Sm_0.4Pr_0.6FeO₃ single crystals are located on the connection line of the qAFM frequencies of PrFeO₃ (0.57 THz) and SmFeO₃ (0.42 THz), therefore the frequency of qAFM increases linearly with doping concentration of Sm³⁺ ion increasing. The Sm_0.4Pr_0.6FeO₃ crystal undergoes a temperature-induced spin reorientation phase transition at about 160 K. When the crystal temperature is lower than 80 K, a wide band absorption peak of about 0.5 THz appears in the absorption spectrum of Sm_0.2Pr_0.8FeO₃ due to the crystal field effect. Our results show that THz spectral data not only allow us to monitor the quality of rare earth orthoferrite crystals prepared by high throughput and analyze the rare earth elements of the sample, but also improve the ability to analyze the physical properties of the co-doped RFeO₃.