The　present　work　envisages　the　development　of　a　novel　and　low-cost　self-mixing　interferometry　(SMI)　technology-based　single　particle　sensing　system　in　a　microchannel　chip　for　real　time　single　micro-scale　particle　sizing.　We　proposed　a　novel　theoretical　framework　to　describe　the　impulse　SMI　signal　expression　in　the　time　domain　induced　by　a　flowing　particle.　Using　Hilbert　transform,　the　interferometric　fringe　number　of　the　impulse　SMI　signal　was　retrieved　precisely　for　particle　size　discrimination.　For　the　ease　of　particle　sensing,　a　hydrodynamic　focusing　microfluidic　channel　was　employed　by　varying　the　flow　rate　ratio　between　the　sample　stream　and　the　sheath　liquid,　and　the　particle　stream　of　a　controllable　width　was　formed　very　easily.　The　experimental　results　presented　good　agreement　with　the　theoretical　values,　providing　a　300　nm　resolution　for　the　particle　sizing　measurement.