Parents of children with ongoing cancer treatment are exposed to risks of developing posttraumatic stress symptoms (PTSS), but little is known about the prevalence and predictors of PTSS among Chinese parents of children with cancer. This study aimed to examine the predictors of PTSS, and explored the correlation of depression, resilience, and family functions with severe PTSS. This cross-sectional survey was conducted from May 2014 to September 2015 among the parents of cancer children treated in four general hospitals in South China. PTSS in the parents were measured using post-traumatic stress checklist-civilian version (PCL-C). Multiple regression analyses were performed to evaluate the predictive values of depression, resilience, family functioning, and the demographic variables for severe PTSS. A total of 279 parents (192 mothers and 87 fathers) participated in the survey. Severe PTSS, as defined by a PCL-C score >= 50, were reported in 32.97% (n = 92) of the total participants, 26.44% (23/87) in the fathers and 35.94% (69/192) in the mothers. The level of PTSS was positively correlated with depression (r = 0.782, P < 0.01) and a poor general family function (r = 0.325, P < 0.01) and negatively correlated with resilience (r = -0.236, P < 0.01). Multivariate analyses indicated that depression, general family function, gender, and education level were significant predictive factors of severe PTSS in the overall parents, accounting for 64.2% of the variance in the prediction of PTSS (R (2) = 0.642, F = 122.602, P = 0.000). For the mothers, depression and family function accounted for 66.5% of the variance in the prediction of PTSS (R (2) = 0.665, F = 187.451, P = 0.000); for the fathers, depression and educational level accounted for 58.8% of the variance in the prediction of PTSS (R (2) = 0.588, F = 59.829, P = 0.000). Parents, especially the mothers, of children with ongoing treatment for cancer are at risk of developing PTSS. Supportive psychological interventions to attenuate the negative emotions of the parents and improve their family functions are important means to promote their natural protective mechanisms to cope with the stressful events.