In　this　paper,　polymer　solar　cells　with　a　tandem　structure　were　investigated　and　optimized　using　a　multiscale　simulation　scheme.　In　the　proposed　multiscale　simulation,　multiple　aspects　-　optical　calculation,　mesoscale　simulation,　device　scale　simulation　and　optimal　power　conversion　efficiency　searching　modules　-　were　studied　together　to　give　an　optimal　result.　Through　the　simulation　work,　dependencies　of　device　performance　on　the　tandem　structures　were　clarified　by　tuning　the　thickness,　donor/acceptor　weight　ratio　as　well　as　the　donor-acceptor　distribution　in　both　active　layers　of　the　two　sub-cells.　Finally,　employing　searching　algorithms,　we　optimized　the　power　conversion　efficiency　of　the　tandem　polymer　solar　cells　and　located　the　optimal　device　structure　parameters.　With　the　proposed　multiscale　simulation　strategy,　poly(3-hexylthiophene)/phenyl-C61-butyric　acid　methyl　ester　and　(poly[2,6-　(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)])/phenyl-C61-butyric　acid　methyl　ester　based　tandem　solar　cells　were　simulated　and　optimized　as　an　example.　Two　configurations　with　different　sub-cell　sequences　in　the　tandem　photovoltaic　device　were　tested　and　compared.　The　comparison　of　the　simulation　results　between　the　two　configurations　demonstrated　that　the　balance　between　the　two　sub-cells　is　of　critical　importance　for　tandem　organic　photovoltaics　to　achieve　high　performance.　Consistency　between　the　optimization　results　and　the　reported　experimental　results　proved　the　effectiveness　of　the　proposed　simulation　scheme.　©　2017　Wei　et　al.