To investigate the heterogeneous effects of Diagnosis-Intervention Packet (DIP) payment reform on the quantity distribution and cost patterns of high/low-complexity cases (measured by Case Mix Index, CMI), and to reveal hospital hierarchical response mechanisms under cost-control pressures, thereby providing policy recommendations.

Using three-year pre-and post-DIP pilot data (2018-2023), we constructed a difference-in-differences (DID) fixed-effects model with medical insurance patients as the treatment group and non-insured patients as the control group. The model incorporated diagnosis-related group (DRG) fixed effects and a policy×high-CMI interaction term to quantify heterogeneous impacts across complexity levels.

The DIP policy significantly reduced per-case medical costs (β₃=-803.570, P<0.001), per-case medical service costs (β₃=-128.434, P=0.019), and per-case material costs (β₃=-290.519, P<0.001) in the treatment group, alongside a reduction in the absolute deviation from the mean by 228.644 CNY (P=0.016) and a shortened average length of stay by 1.080 days (P<0.001). Stratified by CMI, high-CMI cases exhibited additional reductions in per-case costs (β₃=-309.400), material costs (β₃=-549.843), and hospitalization days (β₃=-0.715) compared to low-CMI cases, yet showed an increase in per-case medical service costs (β₃=273.834).

Hospitals adopted distinct response mechanisms under DIP-driven cost constraints. For high-CMI cases, cost control was achieved through technology-for-materials substitution (prioritizing technical service income while reducing material expenditures), whereas low-CMI cases relied on comprehensive cost suppression.These findings highlight the need for complexity-tiered payment adjustments to balance cost containment and care quality.