To obtain calculation formula of lateral load distribution for multi-box steel-concrete composite girder bridges, the traditional rigid transverse beam method, modified rigid transverse beam method and rigid connected beam method are modified for the multi-box composite girder bridges by considering the interface slippage’s effect. The results obtained by the modified theoretical algorithms are compared with the values gained by finite element method which has been validated by experiments. The results show that the modified rigid connected beam method considering slippage effect is applicable to calculate the lateral load distribution of mid-span cross section in multi-box composite girder bridge. When the bridges meet the narrow bridge conditions, i.e. the bridges with span-width ratio is not less than 2, the modified and simplified rigid transverse beam method considering slippage effect is also suitable to calculate the lateral load distribution of the mid-span in the bridges. Finally, it is suggested that the partial load effect of the bridge could be reduced by using incomplete shear connection form or properly strengthening the transverse connection rigidity among steel box girders when the multi-box composite girder bridges are designed.