The invention provides a rapid detection method for disengagement degree of a gap and a liquid injection device, belonging to the field of steel rail detection equipment. The rapid detection method for the disengagement degree of the gap is used for detecting the disengagement degree of a gap between track structural layers and comprises the following steps: measuring the width b of the gap; alongthe length direction of the gap, embedding a liquid injection pipe into the interior of the gap, and sealing the gap along the length direction of the gap, wherein an observation hole is also arranged above the gap; filling the interior of the gap, after liquid flows out the observation hole, stopping filling, and metering volume V of the liquid injected into the gap; calculating the area S1 of the gap according to a formula that S1=V/b; and calculating the disengagement degree N of the gap by virtue of the area S of a track board according to a formula that N=S1/S*100%. By adopting the rapiddetection method for the disengagement degree of the gap and the liquid injection device, detection accuracy can be improved, and the cost of manpower and material resources can be reduced.