The invention discloses an improvement method of soil ecological depth in a moderate saline-alkali land. The improvement method comprises the steps that one, area surface sundries and weeds of the saline-alkali land to be improved are cleared; two, composite biological ameliorant is spread on the surface of the saline-alkali land to be improved and mixed with sand, carbide slag and potassium humate, a large quantity of soil beneficial microorganisms are bred in the short term, and soil activity is rapidly activated; three, the saline-alkali land to be improved is subjected to ploughing, leveling, irrigation, and film covering, then heat preservation and humidification for 10-15 days, acid-base balance is regulated while the soil salinity and alkali content is reduced, so that the microorganisms are over propagated and suffocated to death, and a sterile soil environment is created; four, membrane is removed and soil is air-dried, fermented organic fertilizer is applied, then salt-tolerant plants are sown to improve the biological depth of soil; five, when the growth of the sown salt-tolerant plants reaches the maximum, the salt-tolerant plants are removed and moderate saline-alkalisoil improvement is completed. The method is low in improvement cost, good in effect, and ecological and environmentally friendly.