To address the parameter design challenges of gob-side entry retaining in gangue filling ultra-long workfaces, comprehensive research methods including theoretical analysis, mechanical calculations, and numerical simulation were adopted. An innovative box-form filling body casting technology for gob-side entry retaining was proposed. The technical and economic advantages and disadvantages of six gob-side entry retaining methods were analyzed, and key technical parameters for gangue filling ultra-long workfaces were optimized. The results show: theoretical calculations determined the reasonable width of the roadside support body as 2.94 m with a strength of 8.08 MPa; simulations revealed that when the roadside support body width exceeds 3 m, the gob-side entry retains over 85% of its original cross-section, effectively controlling deformation and failure, showing high consistency with theoretical results and verifying parameter accuracy; quantitative analysis indicates the roadside support body width has significantly higher influence weight on surrounding rock deformation control than gangue filling ratio; as gangue filling ratio increases, the cross-section shrinkage rate first stabilizes then decreases, with no significant change observed when gangue filling ratio is below 75%.