In this article we discuss the question of extravagance or thriftiness in people's consumption in a society of economy using the methods of income elasticity of demand,consumptive elasticity of natural capital to commodity consumption and income elasticity of natural capital consumption. And display the change of people's consumption from extravagance to thriftiness from traditional society to economy society through the Engle's curve of natural capital consumption.It is proposed that in a society of economy,a rational choice is to pursue extravagance from the perspective of income elasticity of demand,while to pursue thriftiness from the perspective of consumptive elasticity of natural capital vs commodity consumption and the income elasticity of natural capital consumption.