Objectives The influence of the variable sized tandem square cylinders（with a side length ratio of 1.5）on the flow field and drag force of cylinders was researched for the flow past tandem square cylinders in a constrained boundary under different gap ratios at a Reynolds number of 100.

Methods Based on the lattice Boltzmann method with the bounce-back boundary condition, comparisons of the effect of equal and variable sized tandem cylinders were made under different gap ratios.

Results The results indicate that variable sized cylinders could accelerate the change of the flow field pattern to a certain extent. However, variable sized tandem cylinders could delay the change of the flow pattern at a gap ratio of 6. In addition, variable sized tandem cylinders, could achieve at most 9.16% reduction in resistance compared with the flow past a single square cylinder. However, the equal sized tandem cylinders could achieve 7.76% at most. On the other hand, compared with the equal sized tandem cylinders, variable sized tandem cylinders could achieve 67.79% reduction in resistance. Variable sized tandem cylinders could obtain better reduction in drag force, so it is a feasible way to achieve flow control and drag reduction.

Conclusions The results in current paper can be served as a basic guidance for the optimization of drag and flow control for the flow past square cylinders in constrained boundaries.