A novel pseudo-random preamble aided timing estimation via arc-tangent operation for CO-OFDM was proposed to tackle with the problems of lower SNR and more synchro errors under the context of transmission dispersion existing in traditional algorithm. The Awoseyila's algorithm could be optimized via the proposed method. Moreover, with using the line of ES-CAZAC preamble for reference, the arc-tangent pseudo-random preamble was proposed. Simulations were showed that the proposed method had much lower estimation error than traditional methods.