The common measurement method of rubber tensile Young' s modulus is the optical lever one. By means of splitting pointed intervention，small diameter variation of the rubber rod under action of an external force is measured. The material of the Poisson's ratio，diameter variation and the function relation between Young's modulus reduce the random error through the experiment many times measurement data. The best estimate of the uncertainty for 　　calculating the Young' s modulus is analyzed. The experimental results show that theoretical error of the present method is around 1.5%，the maximum relative uncertainty is about 1.2%.