Terahertz (THz) transient has become an effective method to study the optical and electronic spin characteristics of the rare earth orthoferrites RFeO ₃. High-throughput grown crystal sample is sliced at different locations, then the continuously tunable rare earth elements co-doped single crystal Sm _xPr _1–xFeO ₃is 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 _0.8FeO ₃and Sm _0.4Pr _0.6FeO ₃single crystals are located on the connection line of the qAFM frequencies of PrFeO ₃(0.57 THz) and SmFeO ₃(0.42 THz), therefore the frequency of qAFM increases linearly with doping concentration of Sm ³⁺ion increasing. The Sm _0.4Pr _0.6FeO ₃crystal 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 _0.8FeO ₃due 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 RFeO ₃.