To address the ambiguous characteristics and undefined application effects of active support adopted in soft rock large-deformation tunnels, the support mechanisms of timely support, active support, high-strength support and support pressure diffusion are explored using theoretical analyses and numerical methods. Then, numerical simulations are carried out to analyze the supporting effects of six typical support schemes under different surrounding rock squeezing degrees, namely timely-high-strength-active support, timely-active support, high-strength-active support, single active support, high-strength-passive support, and passive support. The main conclusions are drawn as follows: (1) The core active support form suitable for soft rock large-deformation tunnels is timely-high-strength-active support, which integrates timely support and high-strength support based on conventional active support and features adequate support pressure diffusion performance. (2) Support pressure diffusion exerts a prominent influence on tunnel support: wider diffusion range of support pressure achieves better control effect on surrounding rock deformation, effectively eliminates weak support zones and improves surrounding rock stability. (3) The arrangement of high-strength support or timely support on the basis of active support markedly optimizes the overall support effect, and effectively improves both the self-bearing capacity and deformation state of surrounding rock. (4) The surrounding rock deformation control and bearing capacity improvement effects of timely-high-strength-active support are superior to the linear superposition effect of timely-active support and high-strength-active support, presenting an obvious “1+1>2” amplification effect. Moreover, such amplification effect increases continuously with the rise of surrounding rock squeezing degree.