In this paper, the planar curves are analytical curves. The arc-length of a curve is geometric and intuitive parameter, for G1 Hermite data taken from the endpoints of a given planar curve and the arc-length of the curve, we construct a PH quintic curve which preserves the arc-length of the curve. The PH curve is the approximation of the planar curve and keeps the geometric property of the orignal curve. The scheme is further used to create a planar G1 spline by joining segments of PH quintic curves after the planar curve is subdivided into segments. With the help of Frenet-Serret formulas and Taylor expansion of planar curves, we deduce the power series of approximation error of segment with arc-length. The approximation order with respect to the given curve by this solution is equal to five and approximation order with respect to its offsets is four. Some related coefficients of terms are listed.