High-Tc YBa2Cu3O7-δ (YBCO) ultrathin films are crucial for both fundamental research and device applications, providing the foundation for high quality high-Tc superconducting single-photon detectors and focused He-ion-beam-etched Josephson junctions, and enabling the construction of functional interfaces with other quantum materials. Herein, We systematically investigated the fabrication of high-Tc YBCO ultrathin films and established a highly reliable strategy. Without using any other copper oxides as buffer or capping layers, this approach successfully reduced the minimum thickness for achieving a critical transition temperature above 80 K (Tc0 > 80 K) to 3 unit cell, representing a significant breakthrough. During this process, three key findings were revealed. First, Tensile strain induced by cubic substrates drives the films toward an underdoped phase, thereby suppressing Tc. Second, besides substrate strain, the surface condition is another crucial factor for achieving high Tc, as evidenced by the significant Tc improvement observed after depositing an ultrathin STO buffer layer on NdGaO₃ substrates. Third, Enhancing the heat transfer efficiency from the heater to the substrate—achieved by adding thermal insulation layers to the substrate—is highly effective in elevating the Tc. Our findings provide clear guidelines for the further improvement of YBCO ultrathin films.