With the growing development of film capacitors in various applications, the requirements for polymer dielectrics have increased accordingly. In this work, a series of ester-cotaining polyimide (EPI) dielectrics were designed and fabricated. Futhermore, integrated exploration of experimentation and molecular simulation is proposed to achieve polymer dielectrics with advanced comprehensive performance, as well as to analyze the dielectric mechanism in-depth. The EPIs show superior thermal resistance and dielectric properties. A Weibull breakdown strength of 440–540 MV m⁻¹, permittivity of 3.52–3.85, dissipation factor of 0.627–0.880% and theoretical energy density of 3.13–4.90 J cm⁻³ were obtained for the EPIs. The relationship between microscopic parameters and dielectric behavior was investigated in detail. According to the experimental and calculated results, there is close correlation between dipolar moment density (μ/V_vdw) and dielectric permittivity (ε_r). It is deduced that the integrated research of experiments and molecular simulation would be an effective strategy to reveal the dielectric mechanism as well as assist in the molecular design of polymer dielectrics.