A fiber humidity sensor based on Fiber-Bragg Grating (FBG) sandwiched in single-mode-multimode fiber core-single mode (SMS) fiber structure is proposed and demonstrated. When the surrounding humidity changes, the central wavelength of FBG remains unchanged for it is insensitive to humidity, while the interference spectrum of SMS fiber structure will shift for it is sensitive to the surrounding humidity. Hence, the shift of the SMS fiber structure interference spectrum with humidity could modulate the FBG core mode. Through measuring the reflected power of the FBG core mode the detection of humidity can be realized. The beam propagation of the SMS fiber structure with different lengths of multimode fiber core (MMFC), diameters of MMFC, and surrounding refractive indices are theoretically simulated with beam propagation method. Theoretical simulation indicates that the output core mode power coefficients shift with surrounding humidity of the SMS fiber structure. Experimental results show that the sensor has a linear response to humidity with enhanced sensitivity of 0.06 dBm·(%RH)-1 in the humidity range of 45%~95%RH with length of 35 mm and diameter of 85 μm. The temperature effect of the sensor is also discussed, the temperature sensitivity is 0.008 nm·℃-1 in the temperature range of 20~80 ℃ and the measurement error of temperature is 0.047% RH·℃-1. Such cost-effective, high sensitive, and reflective power detection based optical fiber humidity sensor could be used in humidity sensing applications.