The high-field-side mid-plane single null (HFS-MSN) divertor configuration on J-TEXT features a special structure symmetric about the midplane, yet experimental observations still reveal up–down asymmetric parameter distributions between the upper and down targets. To investigate the underlying mechanisms of this asymmetry, experiments were conducted under typical sheath-limited discharge conditions, and the experimental phenomena were quantitatively reproduced using the SOLPS-ITER. Both experimental and simulation results indicate that the drift and diamagnetic drift jointly determine the up–down asymmetry. Under the coupled effects of these two mechanisms, the down target exhibits higher peak density, slightly lower temperature, and heat flux, while the upper target shows slightly lower peak density but narrower broadening, with higher and more peaked heat flux distribution. As density increases, the radial drift replaces the poloidal drift as the dominant factor, shifting the density peak toward the far scrape-off layer on the down target. Unlike the main plasma, the asymmetry in C impurity distribution is more pronounced Therefore, this paper also preliminarily explores the reasons for the higher carbon impurity density and stronger radiation in the down divertor. This work presents detailed experimental and simulation results of the unique up–down asymmetry in the HFS-MSN divertor configuration, providing valuable insights for in-depth investigations into the physical mechanisms of divertor asymmetry.