The　stability　of　deploying　cantilever　beam　in　the　uniform　speed　case　is　studied.　The　governing　equations　are　solved　numerically　to　obtain　the　tip　displacement　of　the　deploying　cantilever　beam.　Two　types　of　methods　to　determine　the　stability　of　such　a　time-varying　parameter　system　are　proposed,　namely,　the　instantaneous　eigenvalue　method　and　the　stiffness　matrix　method.　The　eigenvalues　are　obtained　to　analyze　the　instability　by　means　of　studying　the　real　and　imaginary　parts　of　the　eigenvalues.　The　negative　stiffness　occurs　while　the　beam　in　its　unstable　state　and　the　features　of　instability　also　could　be　reflected　by　the　changes　of　stiffness　with　time.　Taking　one　order　and　five　order　truncation　for　the　governing　equations　respectively,　we　compare　the　results　of　the　two　cases　and　find　that　their　results　have　a　good　agreement.　©　2019,　Editorial　Department　of　JVMD.　All　right　reserved.