At present, optical extensometers based on field-of-view splitting (FOV-splitting) technique can only improve the strain measurement accuracy in a single direction, but the measurement accuracy in the perpendicular direction is not high enough, that is, it cannot achieve biaxial high-precision strain measurement at the same time, which limits its application range. Therefore, a biaxial FOV-splitting technique is proposed, which is formed by rotating four identical rhombic prisms, and by combining with the telecentric lens, a biaxial high-precision optical extensometer was developed. The cyclic loading experiment of stainless steel specimen was carried out to verify the superiority of the above optical extensometers over ordinary optical extensometers. In addition, four uniaxial tensile experiments were carried out on stainless steel specimens, and the maximum errors of elastic modulus and Poisson's ratio obtained by the bidirectional optical extensometer and electrical measurement method were 0.62% and 2.86%, and the four average errors were 0.41% and 1.38%, respectively, indicating that this optical extensometer has high strain measurement accuracy.