  Tensor reduction is important for multi-loop amplitude calculation. And the
projection method is one of the most popular approaches for tensor reduction.
However, projection method could be problematic for amplitude with massive
fermions due to the inconsistency between helicity and chirality. We propose an
approach to extend the projection method to reduce the loop amplitude
containing fermion chain with two massive spinors. The extension is achieved by
decomposing one of the massive spinors into two specific massless spinors,
"null spinor" and "reference spinor". Then the extended projection method can
be safely implemented for all the processes including the production of massive
fermions. Finally we present the tensor reduction for the virtual Z boson
decaying to top-quark pair to demonstrate our approach.
