A physics-based microwave emission model for layered vegetation (MEMLV) is developed to simulate the vegetation optical depth (VOD) and scattering albedo of coniferous forests from P- to Ka-band (0.4 GHz-37 GHz). This theoretical study aims to use physics-based forward modelling to guide and support multi-frequency VOD retrieval. The MEMLV consists of three major components: (i) the single-layer discrete scatter model (SL-DSM) that calculates the VOD and single scattering albedo of a single layer, (ii) a new Tree Structure Model (TSM) that represents forests by stratified multi-layer media, and (iii) the Two-Stream microwave emission model (2S-MEM) that combines SL-DSM and TSM to calculate the brightness temperature of forests and their corresponding effective VOD, τ_eff, and effective scattering albedo, ω_eff. Simulation of an exemplary coniferous forest shows that τ_eff increases as frequency increases until reaching saturation at K-band. Meanwhile, ω_eff increases rapidly at low frequencies until reaching a peak at S-band, then decreases until at X-band, and finally increases monotonically as frequency increases. Notably, ω_eff is negligible at P-band for most cases. Sensitivity analyses demonstrate the saturation of τ_eff when canopy height is substantial, and the decreasing trend of ω_eff as canopy height or the areal fraction of canopy gap increases. The MEMLV has the potential to improve the parameterization of retrieval algorithms and to enhance the understanding of the retrieved VOD over a wide frequency range.