The invention relates to a method for measuring relative displacement of butt-joint pieces, in particular to a method for measuring relative displacement of butt-joint pieces through laser displacement sensors. The problem that an existing method is high in cost and complex in operation is solved. The method comprises the following steps that (1) a first reflection plane perpendicular to the butt-joint axis, a second reflection plane and a third reflection plane are arranged on one butt-joint piece, a 45-degree angle is formed by the first reflection plane and the second reflection plane, a 45-degree angle is formed by the first reflection plane and the third reflection plane, and the intersecting line of the second reflection plane and the first reflection plane is perpendicular to the intersecting line of the third reflecting plane and the first reflection plane; (2) the three laser displacement sensors are arranged on the other butt-joint piece and irradiate the three reflection planes respectively, light emitted by the three sensors is parallel to one another and is perpendicular to the first reflection plate, and sensor readings are zeroed; (3) after relative displacement is carried out on the butt-joint pieces, the readings L1, L2 and L3 of the three sensors are read; (4) the relative displacement LX, the relative displacement LY and the relative displacement LZ of the butt-joint pieces are obtained, wherein LX=L3-L1, LY=L2-L1 and LZ=L1. The method has the advantages that the cost is lowered; the operation is easy and convenient, and computation is easy; the error-tolerant rate is high.