In Pakistan，the industrial power supply is insufficient，and large container ports use diesel gensets as their main power source. However，the conventional medium-speed diesel gensets fail to withstand a large suddenly applied load and have low efficiency. To solve this problem，this paper designs diesel gensets with flywheel energy storage，and completes the performance tests. The results show that the proposed diesel gensets can withstand a large suddenly applied load by itself. During its grid-connected operation with a conventional genset，the proposed diesel genset can also improve the work efficiency of the conventional diesel genset. Moreover，the conventional diesel genset can bear a more significant portion of the suddenly applied load，which is not exactly proportional allocation as in the theoretical analysis.