The present invention discloses an apparatus for quickly detecting the length of a spring, comprising a U-shaped tube, a pressure spring and a piston, wherein the U-shaped tube is divided into two vertical sections and one horizontal section having the same internal diameter, the piston is in sliding connection with one vertical section of the U-shaped tube, the outer wall of the piston is fit with the inner wall of the U-shaped tube, the other vertical section of the U-shaped tube is provided with scales, the pressure spring is arranged in the U-shaped tube, one end of the pressure spring is connected with the piston, the other end of the pressure spring is connected with the bottom of the inner wall of the U-shaped tube, in a natural state, the upper end of the piston is parallel and level with the upper end of the vertical section of the U-shaped tube. Through adoption of the apparatus of the present invention, detection time is short, and data measurement is more accurate.