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We have studied the influence of the density of the annihilation region on the positronium continuum.A relation between the ratio 3γ/2γ and the density is explicitly given,with which one can derive directly from the observed 3γ/2γthe density where the annihilation occurs.A unique solution may be found from the observed width of the 0.511 MeV line.We applied the method to three flares observed by GRS/SMM.It is shown that due to the measuring uncertainties in the 0.511 MeV line width,we cannot distinguish a chromospheric source from a coronal source,though both accurately localized.To improve the measuring accuracy of the 0.511 MeV line and the ratio 3γ/2γ will be an important step for a better understanding of the annihilation process in solar flares.