Aiming at the high cost and inconvenient operation of the wavelength demodulation fiber Bragg grating(FBG) anemometer system, an intensity interrogated optical fiber thermal anemometer based on the chirp effect of FBG is proposed. After using a pump laser to heat the 15 mm FBG non-uniformly, it produces a noticeable chirp effect. Under the airflow, the chirp effect of the chirped FBG weakens as its temperature gradient decreases, resulting in reflected optical power decline. Based on this principle, the FBG wind speed measurement with intensity demodulation is successfully realized. The experimental results show that the measurement range of the anemometer is 0~11 m/s, and the sensitivity is up to -28.60 μW/(m·s-1) when the wind speed is 0.1 m/s. In addition, the dynamic response time is less than 1 s (about 0.3 s for low to high wind speed, and 0.9 s for high to low wind speed).