To provide a silver halide color diffusion transfer film unit which attains rapid image formation, ensures less color mixing and gives high blackness, and to provide an image forming method.

The color diffusion transfer film unit comprises: a photosensitive sheet having, on a first transparent support, an image receiving layer, a white reflective layer, a light shielding layer, at least three silver halide emulsion layers having mutually different color sensitivities, and at least two color-mixing preventing layers located between the silver halide emulsion layers and having a non-diffusible reducing agent; a transparent cover sheet having, on a second transparent support, a neutralization layer and a neutralization timing layer; and a body containing an alkali processing composition spread between the photosensitive sheet and the transparent cover sheet, wherein a negative silver halide is contained in the photosensitive sheet, a total coating weight of silver halide is ≤0.9 g/m2 (expressed in terms of silver), and the total number of coating mols of the silver halide is 5-10 times that of the non-diffusible reducing agent contained in the color-mixing preventing layer. The image forming method uses the color diffusion transfer film unit.