We present the estimation of solar observation with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). For both the quiet Sun and the Sun with radio bursts, when pointing directly to the Sun, the total power received by FAST would be out of the safe operational range of the signal chain, even resulting in damage to the receiver. As a conclusion, the Sun should be kept at least ∼2° away from the main beam during observations at ∼1.25 GHz. The separation for lower frequency should be larger. For simplicity, the angular separation between the FAST beam and the Sun is suggested to be ∼5° for observations at 200 MHz or higher bands.