Abstract: This paper investigates the influence of high aspect ratio microstructures on the thermal performance of microchannel heat sinks in calcium silicate low-temperature co-fired ceramic (CaSiO₃ LTCC) multilayer substrates. Thermal-fluid-structure coupling simulations were performed using ANSYS Fluent to study the effects of microstructures on velocity field, pressure field and temperature field. Through simulation, the microchannel heat sink model with optimal heat dissipation efficiency was selected.The main conclusions are as follows: With the increase of microchannel aspect ratio, the heat exchange between coolant and heat sink sidewalls becomes more sufficient, leading to lower surface temperature of the heat sink. However, more complex structures result in greater flow resistance of the coolant and higher inlet-outlet pressure drop. When three rows of fins are arranged (with microstructure aspect ratio of 1.6), the heat sink achieves the lowest temperature of only 329.1 K, which is merely 0.7 K lower than that of two-row fins while the pressure drop increases by 42.0%. Compared with single-row fins, the two-row fin configuration reduces temperature by 9 K with only 6.4% increase in pressure drop. The two-row fin structure maintains relatively low pressure drop while ensuring high heat dissipation efficiency, making it the optimal microchannel configuration.