Static optical properties of conventional opaque building envelopes limit their ability to support both summer cooling and winter heating. Here, we report a multispectral synergistic electrochromic regulator (MSECR) integrating dynamic solar control and spectrally selective infrared emission. The all-thin-film ITO/WO₃/LiNbO₃/NiO/ITO device employs asymmetric indium tin oxide electrodes to form a Fabry-Pérot cavity, while Li⁺ intercalation drives an insulator-to-metal transition in crystalline WO₃ to enable reversible infrared modulation. Under a single electrical drive, the cooling mode combines high solar reflectance with selective thermal emission through the 3-5, 8-13, and 16-25 μm atmospheric windows while maintaining low emissivity outside these windows. Conversely, the heating mode enhances solar absorption and suppresses infrared radiative losses, establishing positive solar-infrared synergy (Fig. 1). Idealized thermal-balance calculations attribute up to 19.6% of the cooling contribution to the third atmospheric window. A metallic-backed version achieves a peak solar reflectance modulation of 0.64 at 1.55 μm, whereas peak infrared emissivity modulations reach 0.27, 0.43, and 0.36 at 4.5, 9.4, and 18.5 μm, respectively. The device retains 66% of its initial optical modulation after 7000 cycles and permits back-side color customization. Outdoor tests under arid desert conditions reveal a maximum chamber-temperature difference of approximately 5.5℃ between heating and cooling modes. EnergyPlus simulations across 11 representative cities predict annual HVAC energy savings of up to 22.19 MJ m^-2, particularly in cold climates. This work establishes a strategy for climate-adaptive, all-season thermal management of opaque building envelopes.