To solve the problems related to the displacement measuring devices in the static load tests of foundation piles，such as small measuring ranges，necessitating datum beams and the critical demands for perpendicularity，the differential pressure connected pipe displacement meter based on liquid-gas coupling formed of connected pipes and differential pressure sensors is proposed，based on the combined use of theoretical analysis，comparative tests and field tests. In the design of the composition of the displacement meter，a kind of liquid level elevation control method and structural formation were chosen，by considering that the liquid surface can automatically look for equal elevation and keep remaining in a horizontal plan under the action of gravity，which allows the initial liquid level elevations at the two ends of the displacement meter to keep the same and reduce errors. The comparative testing within the displacement range of 100 mm demonstrated that the data gained from this displacement meter were in compliance with those gained by dial indicators and stepping motor control type mobile platform，with a maximum absolute error of 0.04 mm. In the field tests for the trial pile project of a Yangtze River Bridge，the maximum downward displacement measured was 36.46 mm，and the maximum upward displacement 127.34 mm，which were basically in agreement with those measured by relative devices，with a maximum difference of +0.28 mm. The matched application procedure to the displacement meter developed were compiled by Labview，supporting the manual intervention short-term measurement and automated continuous long-term measurement. No datum beams were needed during the testing period of the displacement meter. The displacement meter has no requirements for perpendicularity in the installation，which has low costs and great environment adjusting ability and easy to operate. Therefore，it has good publication value in the static load tests for foundation piles.