Minicomposite, as the elementary scale of the full-scale ceramic matrix composites (CMCs), has been commonly considered as the appropriate specimen type for basic research on environmental behavior of CMCs due to its simple structure, low fabrication cost and mature modeling foundation. However, its size is too small relative to the full-scale CMCs, resulting in the existing environmental performance testing equipment developed for macrocomposites not matching it in all respects. In order to fill this gap, a compact environmental performance testing apparatus, which consists mainly of a two-zone furnace, gas humidifier, control system and a pair of quartz tubes and high temperature specimen grips, has been designed and built here. Creep test of SiC fiber at elevated temperatures from 800 to 1100 °C with water vapor and oxidation test of SiC/SiC minicomposite at 1316 °C in stress-free wet oxygen environment were carried out respectively to verify the accuracy and reliability of the rig. With the apparatus, the environmental degradation mechanism of CMCs can be quantitatively analyzed, which lays a foundation for the service life prediction and environmental performance optimization of CMCs.