DNA polymerase θ (POLQ) is a member of the family A DNA polymerase, and is highly expressed in tumor cells and some normal human tissues. POLQ plays a critical role in maintenance of the chromosomal stability in human cells which thereby avoids the occurrence of diseases through its participation in DNA damage tolerance processes. Meanwhile, it is always the culprit for the increased mutation susceptibility of the tissue cells due to its characteristic low-fidelity, for either its abnormally high expression or defect can cause the occurrence of tumors or other diseases. This paper presents the recent progress concerning the mechanism underlying the POLQ involved DNA damage tolerance and its low-fidelity induced mutation susceptibility as well as its role in the occurrence and development of tumors.