In order to figure out the effect of crossed cables, mechanical model of multi-tower cable-stayed bridge with crossed cables was established. A single main span was selected as research object and part of the crossed cables were treated as vertical springs, mechanical mechanism of crossed cables was studied and an analytic formula for calculating the longitudinal restraint stiffness of the crossed cables was deduced. The study shows that, when the tower deformed in longitudinal direction, dead load of the deck was redistributed in the crossed cables and this led to the cable force change, leading to restraint effect for tower. The restraint stiffness of the crossed cables depends on the length of the cables, projection length in horizontal direction and its axial stiffness. A finite element model of cable-stayed bridge with three towers and four spans was established to verify the formula. The numerical method shows good agreement with the formula, which indicates the formula is effective in estimate the longitudinal restraint stiffness of the crossed cables. Numerical analysis shows that using crossed cables could increase the structural stiffness of multi-tower cable-stayed bridge as well as the tower and girder. The crossed cables play more important roles in increasing the stiffness of the structure if the tower and girder have low bending stiffness.