Spot size is an important factor affecting the interaction between laser and single crystal silicon. Different radius millisecond pulsed laser is used to irradiate single crystal silicon. The effects of laser irradiation on large and small spot size, including temperature rise, damage area and damage morphology, are compared and analyzed. The influence rule of spot size on laser-induced single crystal silicon is determined and its mechanism is analyzed. The results show that the peak temperature of the laser irradiation center point is higher when the spot radius is 0.2 cm than when the spot radius is 0.1 cm; the damage area of single crystal silicon increases with the increase of laser energy density; the damage threshold decreases with the increase of laser spot size, and increases with the increase of pulse width.