In this paper, an asymmetric structure optical fiber sensor is proposed to measure relative humidity (RH). The sensing structure is composed of splicing dispersion compensation fiber (DCF) and coreless fiber (NCF), and two sections of single-mode fiber (SMF) at both ends. Peanut shaped structure is used as a beam splitter at the input side, and the NCF is used as a beam combiner at the output side to form interference fringes. The partial cladding of DCF was etched, and polyvinyl alcohol (PVA) was coated on the etched area to form a hygroscopic film. When the ambient humidity changes, the refractive index and thickness of the hygroscopic film will change, which will lead to the wavelength shift of the resonant dip. The experimental results show that the sensitivity of the sensor is 0.1304 nm/RH% and 0.4452 nm/RH% in the RH range of 55%-75% and 75%-95%, respectively. In order to improve the sensitivity further, the original spectrum data is filtered by fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT), and the high-frequency interference components of high-order mode (LP09) and fundamental mode are obtained, which is superimposed with a simulated signal to form Vernier effect. With the method of virtual Vernier effect, the sensitivity in the RH range of 55%-75% is improved to 2.869 nm/RH%, which is 22 times larger than the original sensitivity, and the sensitivity in the RH range of 75%-95% is improved to 2.64 nm/RH%, which is 6 times larger than the original sensitivity.