The invention relates to an on-line logging method for methane carbon isotopes. According to the method, drilling fluid, a drilling fluid degasser, a gas circuit pipeline, a gas purification module, a pressure control module, a flow control module, a carrier gas generation module, carrier gas, a flow controller and a methane carbon isotope analysis module are included, wherein the drilling fluid is introduced into the drilling fluid degasser and degassed to generate drilling fluid gas; the drilling fluid gas is delivered to the gas purification module through the gas circuit pipeline; the gas purification module, the pressure control module and the flow control module are connected through a gas pipeline; a carrier gas generator is connected with the flow controller through the gas pipeline; and under the action of the carrier gas and under the control of the flow control module and the flow controller, the drilling fluid gas is analyzed by the methane carbon isotope analysis module to get relative content of methane and methane carbon isotope data. The invention discloses an on-line portable carbon isotope analysis method, so that reference is provided for quickly evaluating oil gas on site.