The present invention discloses an optical angle measurement method. The method comprises the following steps of: the step 1, placing an element to be measured; the step 2, transiting a surface normalof the surface to be measured to a first reflection mirror; the step 3, transiting the surface normal of the surface to be measured onto a second reflection mirror; the step 4, performing collimationof the second reflection mirror; the step 5, rotating the second reflection mirror, and measuring the rotation angle through an autocollimation; the step 6, performing collimation of the second reflection mirror; and the step 7, repeating the operations in the step 5 and the step 6 for many times, measuring the rotation angle of the second reflection mirror in order until the reflection of cross-hair, reflected by reflection surfaces of the first reflection mirror and the second reflection mirror, of the autocollimation are stopped when appear in the view field of the autocollimation, and finally calculating the inclined angle of the two surfaces to be measured. The method employs the differential though to allow the employing of the autocollimation to measure the extra view field large angle to become possible and performs non-contact measurement so as not to cause damaging and pollution of surfaces of the elements, and therefore, the reliability safety is good, and the precision ishigh.