In north-western China there are vast area of alkaline soils. The fish ponds of sulfate alkaline type always produce sulfide poison which brings serious damage to fish farming. The sulfate reductions in water and sludge system are experimentally studied. Results are as follows: under certain conditions, the sulfide contents in water are in positive proportion to the sulfate contents; the sulfides in water are mainly the escaping products of the sulfate reduction in pond sludge, the sulfides easily accumulated at pH=7.5—8.5; the sulfate reductions almost not occure at the temperature of 4℃,the sulfides accumulates slowly under temperature 20℃ but fast over 30℃; the easily decomposing organisms speed up the sulfate reductions,in which the action of starches are stronger than sugar but humic acids inhibit it; in 1×10^-4N,the order which oxidants inhibit the sulfate reduction is, C₃O₃N₃Cl₂Na>KMnO₄>FeCl₃>KNO₃, in 1×1O^-3N,FeCl₃>C₃O₃N₃Cl₂Na>KMnO₄>KNO₃; the sulfate reduction is biological reduction.