Abstract: We apply linear Regge trajectory and relatived quark model to revisit the low-lying excited singly-heavy mesons and explain the narrow mass splitting of these excited states to arise mainly from relativistic effect of the light quarks. The P-wave masses of the singly heavy mesons (D, D_s , B, B_s ) with spin-parity J^ + ( J = 0 , 1 , 2 ) have been computed, suggesting the unseen states of the mesons B and B_s to be the 0^ + state with mass 5 659 and 5 788 MeV, respectively. Our calculations show that D_s\text0\left( \text2\;317 \right) contains a mass downshift of around 70 MeV while D_s\text1\left( \text2\;460 \right) has no significant mass shift.