The effect of moisture content on the density and the strength of cement-solidified sea silt road embankment adding phosphate tailing was investigated. The optimum solidifying time was determined based on uniform experimental design method considering the factors of cement content and phosphate tailing content at 5 design levels. The different mix proportions were defined. Utilizing compaction test and unconfined compression strength test, referencing to the fluidity experiment of cement concrete, the optimum moisture content is studied on the basis of theories and tests. The influence factors about the optimum moisture content are discussed.