Abstract Coronal mass ejections are explosive plasma phenomena prevalently occurring on the Sun and probably on other magnetically active stars. However, how their pre-eruptive configuration evolves toward the main explosion remains elusive. Here, based on comprehensive observations of a long-duration precursor in an event on 2012 March 13, we determine that the heating and slow rise of the pre-eruptive hot magnetic flux rope (MFR) are achieved through a precursor reconnection located above cusp-shaped high-temperature precursor loops. It is observed that the hot MFR threads are built up continually, with their middle initially showing an “M” shape and then being separated from the cusp of precursor loops, causing the slow rise of the entire MFR. The slow rise, in combination with the thermal-dominated hard X-ray source concentrated at the top of the precursor loops, shows that the precursor reconnection is much weaker than the flare reconnection of the main eruption. We also perform a 3D magnetohydrodynamics simulation that reproduces the early evolution of the MFR transiting from the slow to fast rise. It is revealed that the magnetic tension force pertinent to “M”-shaped threads drives the slow rise, which, however, evolves into a magnetic pressure gradient-dominated regime responsible for the rapid acceleration eruption.