The invention discloses a ball-type ultralow-temperature mirror surface side supporting mechanism for an astronomical telescope. The ball-type ultralow-temperature mirror surface side supporting mechanism is characterized in that a U-shaped slot attached to the outer cylindrical surface of a circular mirror of the telescope is formed in the periphery of a mirror tube of a primary mirror of the astronomical telescope; the U-shaped slot is full of small steel balls. According to an optimal scheme, the small steel balls of phi 4 mm are adopted. According to the ball-type ultralow-temperature mirror surface side supporting mechanism, the deficiencies that a lever and heavy hammer structure is relatively complicated and large in occupied space, a flexible steel band can be only used in a fixed direction, and a mercury band support cannot be applied at the low temperature in the prior art can be overcome; the astronomical telescope as a side support can work at an ultralow temperature of subzero 85 DEG C in the antarctic region.