The calculation of weak interaction rates plays a very important role in studying nuclear physics and nuclear astrophysics. In this work, we calculate the electron capture rate of ⁶⁶Fe in the framework of shell model. We mainly focus on the contribution of allowed transition and forbidden transition to the total rate. It is found that in some astrophysical environments the forbidden transition is very important in contribution to the electron capture rate, in which the non-unique forbidden transition plays a major role. This is very important for nuclear structures and astrophysics.