Provided in the in the invention is a moving platform posture detecting device of a three-degree-of-freedom planar parallel mechanism. The detecting device comprises a base, a moving platform and twodetecting mechanisms with the same structures. The two detecting mechanisms are respectively arranged at the diagonal line of the rectangular base and are connected with the base by connecting plates.The moving platform has a three-freedom parallel mechanism and is connected with the detecting mechanisms by ball connecting rods. Two first-level guide rails in each detecting mechanism are fixed atthe two sides of one connecting plate respectively; first-level sliding blocks are arranged on the first-level guide rails and the upper ends of the first-level sliding blocks are fixedly connected with a lower plate; and a first-level magnetoscale is fixed at the side of the lower plate side. Two second-level guide rails are fixed at the two sides of the lower plate respectively and are crossedwith the first-level guide rails are; second-level sliding blocks are arranged at the second-level sliding rails and the upper ends of the second-level sliding blocks are fixedly connected with an upper plate; a second-level magnetoscale is fixed at the side of the upper plate side. A slot hole is formed in the center of the upper plate; the head of the ball connecting rod is inlaid into the slotholt; and a flange structure is formed at the other end of the all connecting rod to realize fixed connection with the moving platform. The detecting device has advantages of simple structure, low cost, simple and rapid detection method and high efficiency.