Many cancer patients do not have advance directives (ADs), which may lead to unwanted excessive or aggressive care when patients have lost decision-making capacity. The aim of this study was to investigate knowledge and attitudes of approving ADs and explore factors associated with willing to designate ADs among cancer patients in China. We conducted semi-structured interview method investigating 753 in-patients with cancer in two cancer centers. Of those subjects, none of the cancer patients had an AD. Only 22.4 % (118 of 526) approved ADs. Comparing with the disapproved ADs group, the approved ADs group were more likely to discuss the AD with oncologist or nurse (χ (2) = 180.4, p < 0.001) in the cancer center (χ (2) = 244.1, p < 0.001), and they chose more comfort care (χ (2) = 18.8, p < 0.001). Most of cancer patients in the two groups wanted to die at home (72.8 %, 73.7 %, respectively). The older patients (OR, 1.04, 95 % CI, 1.02-1.07, p = 0.001), female (OR, 0.55, 95 % CI, 0.35-0.88, p = 0.013), with higher education levels (OR, 3.38, 95 % CI, 1.92-5.96, p < 0.001), with religious beliefs (OR, 2.91, 95 % CI, 1.71-4.94, p < 0.001), and with higher scores of ECOG (OR, 1.46, 95 % CI, 1.17-1.82, p = 0.001) were associated with desiring for ADs. Our findings indicate that there was a dearth of knowledge and different attitudes toward approving ADs among cancer patients, and some factors of demographic and clinical characteristics influenced their willing to designate ADs. This research highlights the importance of propagandizing the ADs to the public, especially to the patients, and further discussing with them when the time is ripe.