Abstract: Under certain conditions,airflow over a mountain can force trapped lee waves.Scorer (1949) first introduced the Scorer parameter (l2) into a two-layer model,and lead to the necessary condition for trapped lee waves.Since then,lots of investigations have been based on the two-layer model.How ever,the Scorer parameter in the real at mosphere is complex,and the two-layer model is highly limited on dealing with actual questions.It is well known that the distribution of strat ification (N2) often appears in such a pattern,in which a very smal even negative value in the boundary layer,a larger value in t he midtropospheric stable layer,and a smaller value ag ain in upper t roposphere.If t he horizont al w ind speed change with height is not sharp,the Scorer parameter will satisfy l12≤l32＜l22,l12,l22and l32,are Scorer parameters in the low,middle and upper layers,respectively.Int his paper,at heoretic model of this kind of lee waves was est ablished.Using this model,the theoretical wavelength and the analytic disturbance solution of the trapped lee waves were obtained.Then,the trapped lee wavelength of an idealized three-layer case was calculated,on the base of the work that the sensitivity of the wavelength to Scorer parameters in every layer was analyzed.The result showed that the sensitivity to the Scorer parameter in middle layer was the most remarkable,and the value and sign of that in lower layer were all not very sensitive.In order to compare the three-layer model with the two-layer model,the three-layer case was translated into three two-layer cases by use of different methods.The calculation results showed that the wavelengths of these three cases were very different from that of the three-layer case.It is suggested that the Scorer parameter in lower layer can't be ignored when the trapped lee wavelength is calculated.The calculating streamlines in the three-layer model revealed that the strongest disturbance located in the middle layer,which was consistent with the observational result.In addition,a particular three-layer case was calculated,in which the Scorer parameter of the lower layer was negative.The results showed that the structure of stream line disturbance changes less than that of the former three-layer model.