Aimed at the defect that spectral reflectance space is not a valid primary pigments linear mixing space, three principles of establishing the primary pigments linear mixing space are presented. The manuscripts are divided into three types, transparent pigment continuous tone, opaque pigment continuous tone and halftone (translucent pigment) according to the different optical properties of different original pigments. The linear spectral spaces of real physical domain that can reflect original pigments really for these manuscripts respectively are built up. Experimental verification is carried out taking opaque pigment manuscript as an example. Experimental results show that the pigments mixing linear space according to optical properties of original pigments is a valid linear mixing space, of which the spectral can correctly estimate the numbers of primary pigments and approximate the spectral shape of each primary pigment.