The invention provides a wide span beam deflection deformation measuring method, comprising setting a displacement sensor parallel with an object to be detected, and a displacement sensor perpendicular to the object to be detected at lower ends of the object to be detected; fixing the displacement sensor parallel with the object to be detected, and moving the displacement sensor perpendicular to the object to be detected, wherein the data of the displacement sensor parallel with the object to be detected is the horizontal ordinate value of each data points of a deflection curve, used for determining the X-axis ordinate value of a current measuring point, and the data of the displacement sensor perpendicular to the object to be detected is the longitudinal coordinate value of each data point of the deflection curve; a horizontal ordinate value and a longitudinal coordinate value form a data point on the deflection curve, used for measuring the deflection deformation value measured by the current measuring point; and a computer obtaining and fitting the data measured by the two sensors to draft a continuous deflection curve. The method realizes deflection curve accurate fitting through the two sensors, has the characteristics of low measurement cost and high test precision, and can realize continuous measurement at span intervals.