  From a new perspective, we discuss the thermodynamic entropy of
$n+2$-dimensional Reissner-Nordstr\"om-de Sitter(RNdS) black hole and analyze
the phase transition of the effective thermodynamic system. Considering the
correlations between the black hole event horizon and the cosmological horizon,
we conjecture that the total entropy of the RNdS black hole should contain an
extra term besides the sum of the entropies of the two horizons. In the
lukewarm case, the effective temperature of the RNdS black hole is the same as
that of the black hole horizon and the cosmological horizon. Under this
condition, we obtain the extra contribution to the total entropy. With the
corrected entropy, we derive other effective thermodynamic quantities and
analyze the phase transition of the RNdS black hole in analogy to the usual
thermodynamic system.
