This paper proposes a novel diagnostic method for electron density in high-enthalpy shock tunnel plasmas, which integrates low-frequency (LF) electromagnetic (EM) wave attenuation with computational fluid dynamics (CFD) modeling (abbreviation: LF-CFD method). This method utilizes the attenuation characteristics of low-frequency electromagnetic (LE EM) waves (7 MHz) propagating through plasma to construct an objective function. By employing the particle swarm optimization (PSO) algorithm, it inversely solves for the ratio of electron density to collision frequency (n_e/υ_e). Subsequently, the electron density was decoupled using the collision frequency obtained from the CFD simulations. Diagnostic validation experiments were conducted in the JF-10 high-enthalpy shock tunnel. The results demonstrate that the LF-CFD method aligns well with electrostatic probes during stable flow periods, with a maximum error of less than 0.5 orders of magnitude, thereby validating the effectiveness of this method. The study also indicates that an operation frequency of 7 MHz is suitable for most high-enthalpy shock tunnel plasma scenarios. However, for electron density below the order of 10¹⁷m^-3, a higher operating frequency is required to enhance the diagnostic accuracy. The proposed method offers advantages such as non-contact measurement, system simplicity, and adaptability to complex experimental environments. Nonetheless, its diagnostic accuracy depends on the precise evaluation of the collision frequency and plasma thickness, making it most applicable to plasma sources with uniformly distributed collision frequencies over time. This study provides a novel and reliable diagnostic method for ground-based experiments on the EM characteristics of high-speed target plasmas, with the potential for future extension to inductively coupled plasma (ICP) wind tunnels and other scenarios.