Most of the reconstruction-based robust adaptive beamforming (RAB) algorithms require the covariance matrix re-construction (CMR) by high-complexity integral computation. A Gauss-Legendre quadrature (GLQ) method with the highest alge-braic precision in the interpolation-type quadrature is proposed to reduce the complexity. The interference angular sector in RAB is regarded as the GLQ integral range, and the zeros of the three-order Legendre orthogonal polynomial is selected as the GLQ nodes. Consequently, the CMR can be efficiently obtained by simple summation with respect to the three GLQ nodes without integral. The new method has significantly reduced the complex-ity as compared to most state-of-the-art reconstruction-based RAB techniques, and it is able to provide the similar perform-ance close to the optimal. These advantages are verified by nu-merical simulations.