  In this paper, we propose a novel expansion to compute the large interval
limit of the R\'enyi entropy of 2D CFT at high temperature. Via the replica
trick, the single interval R\'enyi entropy of 2D CFT at finite temperature
could be read from the partition function on $n$-sheeted torus connected with
each other along a branch cut. We calculate the partition function by inserting
a complete basis across the branch cut. Because of the monodromy condition
across the branch cut in the large interval limit, the basis of the states
should be the ones in the twist sector. We study the twist sector of a general
module of CFT and find that there is an one-to-one correspondence between the
twist sector states and the normal sector states. As an application, we revisit
the non-compact free scalar theory and discuss the large interval limit of the
R\'enyi entropy of this theory by using our proposal. We find complete
agreement in the leading and next-leading orders with direct expansion of the
exact partition function. Moreover, we prove the relation (\ref{th}) between
thermal entropy and the entanglement entropy for a generic CFT with discrete
spectrum.
