Head-on collision between ion-acoustic (IA) multisolitons propagating in opposite directions in a magnetised spin quantum plasma having inertial ions and degenerate electrons with spin-up and spin-down states is studied. The extended Poincaré–Lighthill–Kuo (PLK) method is adopted to derive two Korteweg–de Vries (KdV) equations. Further, the Hirota bilinear method is employed to determine the soliton solutions of two KdV equations for single-soliton, double-soliton and triple-soliton cases. The analytical expressions of phase shift after head-on collision of IA multisolitons in different cases are also derived. The combined effects of different physical parameters such as spin-density polarisation, magnetic field and other physical parameters on the characteristics of IA solitons,and time evolution as well as phase shifts due to head-on collision between IA multisolitons have been numerically described. It is observed that the presence of spin-density polarisation awfully affects propagation properties of IA solitons and phase shifts for different cases. The results of the present investigation may be useful to understand the nonlinear features of a variety of nonlinear excitations in dense astrophysical plasma regions like white dwarfs.