Analytical model for isochronal phase transformation kinetics attracts much attention for its advantages and importance. However, the simple but exact analytical formula of the isochronal transformation is unavailable because of the so-called temperature integral, and the asymptotic expansions have to be adopted to obtain approximate results. Here a generally used asymptotic expansion was proved divergent, and a reasonable approximation was proposed to obtain a more precise description as compared numerically to the previous one. Based on the proposed approximation, an analytical model for isochronal transformation kinetics was developed, which was proved more effective than the previous analytical model when the transformation occurs in a narrow temperature range and exhibited an identical form to the previous model when in a wide temperature interval.