<p>The mediators and moderators of the effect of mindfulness- and compassion-based interventions (MCBIs) on parental burnout are not yet well understood. This study examined the impact of an MCBI on parental burnout in China, investigated the mediating roles of parenting stress, mindful parenting, and self-compassion, and explored the moderating effects of baseline mindful parenting, baseline self-compassion, and sociodemographic variables. Chinese mothers of preschool-aged children were randomly assigned to either the intervention group (<i>n</i> = 73) or the waitlist control group (<i>n</i> = 74). A 14-day self-help online MCBI was delivered over a three-week period. Assessments were conducted at preintervention, postintervention, and at a 3-month follow-up. Participants completed 62% of the intervention on average (<i>SD</i> = 38%). At postintervention, the intervention reduced parental burnout and parenting stress and improved self-compassion, with small to medium effect sizes. The effect on mindful parenting and the sustained 3-month effect on parenting stress were generally in the expected direction, although these findings were less consistent across analytic approaches. Parenting stress, mindful parenting, and self-compassion mediated the intervention’s effects on parental burnout. Involvement in parenting moderated the intervention’s effect on parental burnout, such that mothers with moderate to high involvement benefited from the intervention, whereas those with low involvement showed limited improvement. Baseline mindful parenting, self-compassion, age, socioeconomic status, the first child’s age and gender, and perceived partner involvement were not significant moderators. These findings suggest that brief online MCBIs may help reduce parental burnout among mothers of preschoolers. Parenting stress, mindful parenting, and self-compassion may be important mediators, while self-involvement in parenting may be an important moderator. This study is not preregistered.</p>