Lake Dianchi has undergone accelerated alternations resulting from human activities during the last 50 years, and provides an exceptional example for the study of regime shift in a shallow lake. Focusing on regime shift, we systematically studied and analysed alternations of key components of its ecosystem, including water parameters, phytoplankton and zooplankton biomass, macrophyte composition and fish populations. We found that the shifting course of Lake Dianchi went through three major phases, corresponding to three distinct states: Phase I clean, stable state during the 1950s; Phase II transitional state during the 1970s-1980s; and phase III stable, turbid state since 1993. We analysed all data and samples, identifying the threshold concentrations at which regime shift occurred: total nitrogen (TN) and phosphorus (TP) reaching 1.2 mg/L and 0.13 mg/L, respectively. Results from this study strongly suggest that the increasing input of external nutrients, reclaiming land from the lake and foreign fish invasion were major inducements to the accelerated shift of Lake Dianchi to its turbid, stable state. These findings provide valuable guidelines for potential efforts to reverse this unnatural process in Lake Dianchi. The restoration may need to: (1) thoroughly and accurately identify pollution sources to control and reduce excessive external nutrients flowing into the lake; (2) return reclaimed land to the lake and recover the littoral zone to its natural state to facilitate increased restoration of macrophytes; and (3) strategically adjust fish populations by reducing benthivorous and filter-feeding fish populations that prey on zooplankton, while enlarging zooplankton populations to control phytoplankton biomass.