Relative humidity monitoring, usually accompanied by temperature perturbation, is an of great research field because of its various applications in the agricultural and industrial fields, and environmental monitoring. In this paper, we skillfully design and fabricate a compact fiber-optic sensor containing of fiber Bragg grating (FBG) and polymer microsphere to monitor humidity and temperature at the same time. Polymer microsphere demonstrates excellent advantages, including good thermal-humidity effect of polymeric adhesive, and the excitation of a large amount of scattered light and whispering-gallery modes in the microsphere to strongly enhance multi-mode interference and fiber-optic sensitivity. Thus, humidity sensitivity of 69.2 pm/%RH and temperature sensitivity of −110.2 pm/oC are achieved based on the polymer microsphere. FBG, another integrated structure, shows temperature sensitivity of 10.6 pm/oC and no response on ambient humidity, which could be utilized as a compensating component to extract humidity information, finally achieving simultaneous monitoring of humidity and temperature.