A length-adjustable mechanical arm includes: a base; a first arm unit having one end formed with a first pivot portion pivoted to the base, and another end formed with a second pivot portion, a second arm unit pivoted to the first arm unit includes a pivot member, an inner sleeve, an outer sleeve and an accessory mounting seat. The length of the second arm unit of the mechanical arm is adjustable by adjusting the inner and outer sleeves, and the second arm unit is switchable between an extended position and a retracted position. Therefore, the mechanical arm can be used for two distance operation.