Recently， spin effect in narrow gap semiconductor heterostructures has attracted much attention. However， Rashba spin effect is quite different in Ⅳ-Ⅵ asymmetric quantum wells (QWs) with various growth orientations due to their multivalley and anisotropic band structures. In this work， we calculated Rashba splitting in Pb1-ySryTe/PbTe/Pb1-xSrxTe asymmetric QWs with growth orientations ［100］， ［110］ and ［111］. The results show that Rashba splitting reaches the maximum of 2.2meV when the well width of PbTe QWs with growth orientation ［100］ is 5.0nm， and two groups of Rashba splitting is obtained in PbTe and QWs with growth orientations ［110］ and ［111］， respectively， because the quantum confinement lifts off the fourfold degeneracy of the L-energy valleys. The dependences of Rashba splitting on asymmetry of QWs， well width， temperature and k∥ (the wave vactor in the plane) are also investigated. Large Rashba spin splitting may make Ⅳ-Ⅵ asymmetric QWs as a material candidate for spintronic devices.