Breast cancer （BC） is one of the most common diseases in women. The accurate detection for BC is very important in clinical practice. In this paper， in-vivo breast cancer in a subcutaneous xenograft mouse was studied with the continuous-wave terahertz （THz） reflection imaging system at 2.52THz. The high reflection region in THz images of in-vivo subcutaneous xenograft mouse corresponded well to the tumor area in visible. The difference value of 15% between tumor and normal tissues was observed. For the gold standard， the tumor areas at different depths from the surface of skin in mouse model were obtained by hematoxylin and eosin-stained images. The tumor areas increased with the increase of depth. In contrast with THz image， it is found that the tumor area in THz image was similar to that in hematoxylin and eosin-stained images at a depth of 460μm. Therefore， the detection depth of THz wave to in-vivo breast cancer in subcutaneous xenograft mouse modelis about 460μm， and THz wave is expected to realize the detection of deep tumors.