An improved structure of ladder-type fold baffle is proposed to block the triangular leakage zones in original heat exchangers with helical baffles. The numerical results showed that the shell-side tangential velocity and radial velocity in improved heat exchanger increase significantly and the shell-side fluid becomes approximately continuous spiral flow. And the configuration of ladder-type fold baffles was optimized. The numerical results showed that the shell-side heat transfer coefficient of the improved heat exchanger increases by 82.8-86.1%, when the folding ratio phi is 0.3, the baffle height omega is 60%, and the folding angle alpha is 37. The associated pumping power penalty is about 21-549W due to the increased shell-side pressure drop. The thermal performance factor TEF enhances by 28.4-30.7%, which demonstrates that the ladder-type fold baffle effectively improves the heat transfer performance of heat exchangers with helical baffles. The results of this paper are of great significance in the optimal design of heat exchanger. (C) 2014 Elsevier Ltd. All rights reserved.