The invention provides biological organic soil obtained through improving foundation pit soil employing a compost product and a preparation method of the biological organic soil. The method comprises the steps of mixing the compost product which is generated through fermenting and decomposing pig manure and garden waste at a certain initial carbon-nitrogen ratio and the foundation pit soil, and then inoculating the mixture into a strain with degraded lignocelluloses to obtain the biological organic soil. The improvement effect is measured through measuring a physical structure, chemical indexes and biological indexes of the biological organic soil; formation of an urban construction waste soil crumb structure is promoted to a certain extent, and the original amount of poor nutrients and microorganisms in urban construction waste soil is improved; a feasible method is provided for resource utilization of the garden waste and urban construction waste soil; and meanwhile, the concept of the biological organic soil is provided on the basis of a landscaping and planting soil standard.