In this paper, we derive a simple formula to generate a wide-sense systematic generator matrix(we call it quasi-systematic) B for a Reed-Solomon code. This formula can be utilized to construct an efficient interpolation based erasure-only decoder with time complexity O(n²) and space complexity O(n). Specifically, the decoding algorithm requires 3kr+r²-2r field additions, kr+r²+r field negations, 2kr+r²-r+k field multiplications and kr+r field inversions. Compared to another interpolation based erasure-only decoding algorithm derived by D.J.J. Versfeld et al., our algorithm is much more efficient for high-rate Reed-Solomon codes.