In this study theoretical expressions are derived to investigate the non-Condon effect for symmetry-forbidden optical transition using displaced-distorted harmonic potential energy surfaces. These expressions can efficiently cope with multipromoting modes and multielectronic states involved in the non-Condon effect at a finite temperature. Ab initio and molecular dynamics calculation results can be directly invoked into the formulas. Based on the proposed formulas, the temperature dependence of the interference effects of multipromoting modes on the non-Condon optical linear spectra is investigated. To demonstrate the computational formulas, the optical absorption and dispersion fluorescence spectra for the forbidden transition of neat acetone. (1)A(1)-(1)A(2)(n-pi*), are also studied. Simulation results indicate that the vibrational frequency of CH3 torsion mode of acetone plays an important role in the optical spectra. Moreover, the electronic energy gap (adiabatic transition), the Stokes shift caused by environmental interaction, and the average Huang-Rhys factor for intermolecular modes in neat acetone are obtained as 29 900 cm(-1) 5400 cm(-1). and 0.607, respectively.