Abstract: A kind of robust pitch estimation algorithm is presented for parametric speech coding with transparent quality at very low bit rate. We adopt improved computation of autocorrelation function for the case of F_O change in the end of sentences, and realize accurate tracking to rapidly changed pitch. Based on decision rules, such as the slow change of pitch between adjacent frames, the improved autocorrelation functions are used to get the robust pitch reference for next steps. Pitch estimation of speech vowel is enhanced. Using conventional pitch track algorithm makes final decision of pitch estimation. Informal tests show that the algorithm could achieve the enhancement of pitch estimation and decrease the perceptual distortion made by improper pitch estimation.