The sustainable use of urban land is a n important part of sustainable deve lopment.It needs quantitative scientific assessment on the system of urban land use so that to measure whether the urban land use is sustain-able.Urban Land use system is a compl icated and largely open one.The field theory analysis on co -ecosystem provides a new concept for the study o n sustainability of urban land use.B ased on separate measurement for the level of sustainable use,potential of sustainable use,harmonious degree of sustainable use,the sustainab le use degree of urban land is assessed by case study of Hangzhou City.The degree of sustainable use of urban land in Hangzhou City and six districts attached is an alyzed and assessed.