Theoretical calculations for Poly(cyclopenta-dithiophene-benzothiadiazole)using TDDFT method were performed.Six different PCPDTBT oligomers(n=1-6)were selected to explore the properties of polymers.To examine the effect of method,benchmark calculations using ADC2 method and TDDFT method including five functionals including BP86(0%HF exchange),B3LYP(20%HF exchange),PBE0(25%HF exchange)and BHandHLYP(50%HF exchange)for monomer were carried out.The properties of four lowest singlet excited states,torsional potential and transition density analysis were investigated.Extrapolation to infinite chain length indicates the probability of hop from the higher excited state S2 to the first lowest excited state S1.The torsional potential energy surface of the central dihedral angle shows that the energy gap between S2 and S1 decreases with the variation of the dihedral angle from 0 to 90 degrees.The present work can provide basis for the dynamical simulation of ultrafast internal conversion from S2 to S1.