The slot flow characteristcs of NHLP 2D multi element airfoil was calculated and analysed within the Reynolds number range from 1×106 to 30×106 and Mach number 0.197. A nominal boundary layer thickness δ/c was defined according to the velocity distribution at the exit of slot. δ/c decreases monotonically as Reynolds number increases. And the decreasing rate of δ/c also decreases with Reynolds number which is in accordance with the effect of Reynolds number on boundary layer. The δ/c defined here can be used in the quantitative research on multielement airfoil boundary layer thickness. For the slat slot, δ/c on main element is higher than that on slat trailing edge. And δ/c changes oppositely on the two region. The flow in flap slot is strongly affected by the down wash of main element. As a result, δ/c on flap hardly changes with incidence. At the range of 1×106≤Re≤2×106, total boundary layer thickness δT/c changes significantly with Reynolds number. At the range of Re≥3×106, δT/c decreases slowly with Reynolds number. Within the scope of current study, δT/c does not enter the prospective area of Reynolds number. The δT/c shows a linear reduction with Reynolds number as Re≥15×106 which provides reference for extrapolation of Reynolds number effect.