A non-charge-sheet based analytical model of undoped symmetric double-gate MOSFETs is developed in this paper using the SPP approach. The essential difference of the present theory compared with the previous lies in that the Poisson equation is solved in the term of the electron concentration rather than the term of the surface potential. This solution formulates electrical field, surface potential in inversion charge term rather than the surface potential. Thus, a non-charge-sheet-based analytical solution of inversion charge is obtained directly replacing solution of transcendent equation groups of the surface potential. The obtained inversion charge relation then serves to develop a non-charge-sheet-based analytical theory for undoped symmetric double-gate MOSFETs from Pao-Sah current formulation. The formulated model has an analytic form that does not need to solve for the transcendent equation as in the conventional surface potentials or Pao-Sah formulation. The validity of the model has also been demonstrated by extensive comparison with AMD double-gate MOSFETs data.