An online measurement method of straightness is studied based on multi-probe optical scanning method. A three-probe scanning method with tilt compensation is proposed. Reconstructed form can accurately express the high frequency elements of the straightness error, and owns better suppression capability of the measurement noise. The measurement principle of the proposed method is introduced firstly. The construction algorithm of the straightness errors is deduced based on the least square solution and the computer simulation is performed. The experimental system is built for verificating the validity of the proposed method. The experimental results show that the proposed method can accurately reconstruct the straightness errors of step form, and the standard deviation is less than 10 micrometer.