The invention relates to a beach saline-alkali soil conditioner with both soil improvement and fertilizer efficiency and preparation and application methods thereof. The soil conditioner comprises according to parts by weight: 80-90 parts of domestic sludge, 1.5 parts of calcium oxide, 3.5-7.5 parts of biochar, and 5-11 parts of crop straw. The technical bottleneck in quickly conditioning beach soil at low cost is effectively solved; by combining the domestic sludge and biochar, it is possible to effectively promote the formation of beach soil aggregates, reduce soil salt return, reduce pH, increase oil organics content and nutrients such as N and P, effectively promote beach saline-alkali soil structural improvement and fertility improvement, and ensure backup resources for agricultural farmlands of China.