As a member of the human epidermal growth factor receptor (HER) family of receptor tyrosine kinases, HER3 is an aberrantly activator of the PI3K/AKT pathway. Studies have indicated that HER3 is related to the progression of a variety of tumor types such as breast cancer, non-small cell lung cancer (NSCLC), ovary cancer and colon cancer, and in acquired resistance to EGFR and HER2 therapies. However, the attempts to target HER3 with neutralizing antibodies are not ideal previously. This is most likely due to the fact that the antibodies targeting HER3 fail to completely block the heterodimerization of HER3 and other receptors. Antibody-drug conjugates (ADCs) can specifically bind to target cells and exert the highly cytotoxicity effect on cancer cells through chemical drugs. ADCs have been widely used in clinical cancer therapies. We analyzed and optimized the structure of the antigen-antibody complex between HER3 and antibody LmAb3 by computer-aided molecular simulation technology, and the key sites involved in antigen binding in LmAb3 were predicted by distance geometry and computer graphics technology. Then a novel anti-HER3 antibody FD001 was obtained by point mutation technology. The affinity measurement by ForteBio results showed that the affinity of FD001 is much higher than LmAb3, the K_D values of FD001 and LmAb3 with HER3 were 1.48E-11 and 2.46E-10, respectively. Antibody drug conjugate FD001-DM1 is obtained by coupling FD001 to DM1 [emtansine, N²'-deacetyl-N²'-(3-mercapto-1-oxopropyl)-maytansine] by lysine coupling technology. The results of cell cytotoxicity experiments showed that FD001-DM1 could effectively inhibit the proliferation of HER3-positive HT-29 colon cancer cells, with EC₅₀ value of 33.62 nmol·L^-1. The in vivo xenografts therapy results showed that the tumor volume of the FD001-DM1 treatment group was about 25% of that of the control group, and there was no significant weight reduction of the mice. These results reveal that FD001-DM1 had good in vivo and in vitro anti-tumor activity with high safety, which may provide effective help for further exploration of HER3-targeted ADCs drugs. The mice in this study were used and treated in accordance with international laboratory animal care and use guidelines and approved by the Animal Ethics Committee of the Military Cognitive and Brain Science Institute of the Military Medical Research Institute.