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The invention discloses a method for testing a deflection / longitudinal displacement change of a bridge based on a four-quadrant position detector. The method is characterized by comprising the following steps: firstly, the four-quadrant position detector is installed in the centre of the bridge or other points to be tested at the bottom of a bridge deck; a laser source is installed on a pier and / or a certain point below the bridge deck through an electric displacement table; then, testing is carried out by matching the four-quadrant detector with the laser source, so that the light spot of laser is over against the centre of the detector; when the deflection of the bridge deck changes, the four-quadrant detector longitudinally moves together with the point to be tested; the light spot position, formed on the four-quadrant detector, of the laser moves oppositely relative to the detector; and different position coordinates of the light spot on the detector before and after the deflection of the bridge changes and light spot position variable quantity on the four-quadrant detector, namely deflection position variable quantity of the point to be tested on the bridge deck can be calculated. The method disclosed by the invention has the advantages of simple structure, good controllability and high precision and is capable of realizing real-time online test of the deflection / longitudinal displacement of the bridge.