The effects of different hygrothermal treatment processes on the hygrothermal aging properties of 25% short glass fiber (GF)-reinforced polyamide 66(PA66) plates were investigated by macroscopic mechanical properties and microscopic characterization methods. The 25% GF/PA66 fixed plates with different hygrothermal treatment processes were put into the same hygrothermal aging environment for periodic sampling and testing, and the analysis results show that the hygrothermal weight gain rate of fixed plates with different hygrothermal treatment processes decreases to different degrees. The mechanical properties results show that the hygrothermal treatment process improves the hygrothermal aging resistance of the fixed plates, and maintains the high tensile and flexural strength, and surface hardness in the hygrothermal aging environment; the temperature change of the hygrothermal treatment process has a greater impact on the decay of mechanical properties. The bending fracture surface was observed by scanning electron microscopy and it is found that after the hygrothermal treatment, the surface of the extracted fibers is covered with a large amount of resin matrix, and the cavities left by the extracted fibers at the fracture are also less, which indicates that the wet heat treatment can maintain good interfacial bond strength between the glass fibers and PA66 matrix of the specimens.