AbstractAll‐polymer solar cells (All‐PSCs) are considered the most promising candidate in achieving both efficient and stable organic photovoltaic devices, yet the field has rarely presented an in‐depth understanding of corresponding device stability while efficiency is continuously boosted via the innovation of polymer acceptors. Herein, a ternary matrix is built for all‐PSCs with optimized morphology, improved film ductility and importantly, boosted efficiency and better operational stability than its parental binary counterparts, as a platform to study the underlying mechanism. The target system PQM‐Cl:PTQ10:PY‐IT (0.8:0.2:1.2) exhibits an alleviated burn‐in loss of morphology and efficiency under light soaking, which supports its promoted device lifetime. The comprehensive characterizations of fresh and light‐soaked active layers lead to a clear illustration of opposite morphological and physical degradation direction of PQM‐Cl and PTQ10, thus resulting in a delicate balance at the optimal ternary system. Specifically, the enlarging tendency of PQM‐Cl and shrinking preference of PTQ10 in terms of phase separation leads to a stable morphology in their mixing phase; the hole transfer kinetics of PQM‐Cl:PY‐IT host is stabilized by incorporating PTQ10. This work succeeds in reaching a deep insight into all‐PSC's stability promotion by a rational ternary design, which booms the prospect of gaining high‐performance all‐PSCs.