Simultaneous ammonium and phosphate recovery could notably improve the dewaterability of alkaline fermented sludge, the mechanisms of which, however, remains unclear at present. The influence of zeta potential, divalent ions, extracellular polymeric substances (EPS), dissolved polymers and struvite were studied in batch experiments. Under the optimal ammonium and phosphate recovery condition [i. e. pH =10.0, n(P)/n(N) = 1.3 mol x mol(-1), n(Mg)/n(N) = 1.9 mol x mol(-1)], it was found that magnesium ion could not only decrease the absolute value of zeta potential to 14 mV, but also reduce the monovalent to divalent ions ratio to 9 mol x mol(-1), which promoted the dewaterability of alkaline fermented sludge; also, the dissolved polymers and EPS, especially the dissolved protein and the loosely bound EPS, reduced remarkably. Results showed that all the factors above promoted sludge dewatering. Furthermore, the struvite formed during ammonium and phosphate recovery improved the dewaterability.