Taking polluted fish ponds in Lake Baiyangdian as the research object，this paper carries out engineering tests for four different ex-situ water quality improvement processes，namely，magnetic coagulation-magnetic separation + ecological pond，magnetic coagulation-magnetic separation + reed wetland，biological ultra-deep purification treatment system，magnetic coagulation-magnetic separation + biological ultra-deep purification system + reed wetland. The results reveal that the chemical oxygen demand（COD）and total phosphorus（TP）removal rates of polluted pond water by the magnetic coagulation-magnetic separation process are 42.7% and 88.66%，respectively. Those by the biological ultra-deep purification treatment system are 60.50% and 95.59%，respectively，and after treatment，the main water quality indexes can reach the standards of Class Ⅲ in GB3838-2002. If the magnetic coagulation-magnetic separation and the biological ultra-deep purification treatment system are combined to treat the polluted fish pond water，the COD and TP removal rates are 62.96% and 94.07%，respectively. Compared with the biological ultra-deep purification system，the combination can further degrade the pollutants in the effluent produced by the magnetic coagulation-magnetic separation process，but the final effluent is not superior to that by the biological system alone. Reed wetlands can improve the effluent quality to a certain extent，and the COD and TP removal rates of the effluent of the whole treatment system are about 16.08% and 33.1%，respectively.