  We present a set of Lorentz invariant kinematic variables for reconstructing
mass of semi-invisible decaying particles pair-produced at lepton colliders,
$m_{\rm RC}^{\rm min}$, $m_{\rm RC}^{\rm max}$ and $m_{\rm LSP}^{\rm max}$,
with analytical formulas. They give the minimal and maximum bounds of the
decaying particle mass and upper bound of the invisible particle mass. In the
search of new physics, these variables can greatly enhance the statistical
significance of signal. For the process of smuon pair production at
$\sqrt{s}=240~{\rm GeV}$ lepton collider of 5 ab$^{-1}$, the cross section
detection limit is pushed by one order, and the expected exclusion and
discovery limits are set above $\sqrt{s}\big/2$ and go into the off-shell
region. Moreover, these variables can also be used to improve the precision of
$W$-boson mass measurement in full leptonic decayed channel. At future lepton
collider, the precision can reach to $2\sim 3$ MeV level.
