The invention provides a linear feed system's guide rail linearity measuring apparatus and method wherein the reference coordinate system of the linear feed system's guide rail uses the vertical direction as Z axis, the moving direction of the sliding block as Y axis, and the direction horizontally perpendicular and away from the guide rail as X axis. The apparatus comprises a first measuring clamp and a second measuring clamp fixedly arranged on the sliding block respectively; a laser collimator fixedly arranged on the first measuring clamp; a No. 2 displacement sensor and a No. 3 displacement sensor symmetrically arranged in the X direction of the second measuring clamp; and a No. 1 displacement sensor on the Z direction of the second measuring clamp. The optical path of the laser collimator is parallel to the Y direction of the first measuring clamp. The center line of the No. 1 displacement sensor passes through the center point of the laser collimator. The three displacement sensors are located on the plane perpendicular to the Y direction. The No. 2 displacement sensor and the No. 3 displacement sensor are used to measure the displacement gap between the guide rail and the Y direction of the sliding block respectively. The No. 1 displacement sensor and the laser collimator are used to measure the displacement gap between the sliding block and the Z direction of the guide rail.