−1.5 −1 −0.5 0 0.5 1 −25 −20 −15 −10 −5 0 w$_{0}$ w a SN SN+BaoR SN+Bao2 SN+BaoZ SN+Bao4 SN+Bao4+wmap (a) −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 −40 −35 −30 −25 −20 −15 −10 −5 0 5 w$_{0}$ w a SN SN+BaoR SN+Bao2 SN+BaoZ SN+Bao4 SN+Bao4+wmap (b) −1.4 −1.2 −1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 −20 −15 −10 −5 0 w$_{0}$ w a SN SN+BaoR SN+Bao2 SN+BaoZ SN+Bao4 SN+Bao4+wmap (c) −1.5 −1 −0.5 −10 −8 −6 −4 −2 0 2 w$_{0}$ w a SN SN+BaoR SN+Bao2 SN+BaoZ SN+Bao4 SN+Bao4+wmap (d) FIG. 1: The marginalized 1 σ contours of w$_{0}$ and w$_{a}$ in the CPL model. The green line is for SNIa, the dashed black line is for SNIa+BaoR, the cyan line is for SNIa+Bao2, the magenta line is for SNIa+BaoZ, the blue line is for SNIa+Bao4, and the red line is for SNIa+Bao4+WMAP5. The SNIa used in (a)-(d) are Csta, Cstb, Cstc, and Cstd, respectively. Om diagnostic is useful in establishing the properties of DE at low redshifts. The constant Om indicates that the DE is the cosmological constant and the bigger value of Om shows that w is bigger [5]. Om ( z ) is less sensitive to observational errors than EOS w ( z ). Due to the degeneracy between Ω$_{m}$$_{0}$ and w , the property of w ( z ) depends on the current value of matter energy density Ω$_{m}$$_{0}$. However, Om diagnostic provides a null test of the cosmological constant without knowing the exact value of Ω$_{m}$$_{0}$. To reconstruct Om ( z ), we need to apply the specific model and model parameters. Following [11], we consider the uncertainties of Ω$_{m}$$_{0}$, w$_{0}$ and w$_{a}$ when we we reconstruct Om ( z ). 7