Background <p>Breast cancer patients and their family caregivers would experience positive changes after diagnosed. We aimed to construct an online positive psychological intervention program for breast cancer patients and their caregivers and evaluate its feasibility and preliminary effects on positive psychological experience and quality of life.</p> Methods <p>A single-arm pretest and posttest quasi-experimental design was used in this study. Thirty-five dyads of chemotherapeutic breast cancer patients and caregivers were recruited. Four modules of online positive psychological interventions were developed, each lasting 20–30&#xa0;min. Participants received a 4-week online positive psychological intervention during chemotherapy. Feasibility was assessed, including the rates of enrollment, retention, adherence and acceptability. Resilience, hope, perceived benefit, positive aspects of caregiving, and quality of life were assessed as the preliminary effects of the intervention. 62.5% of recruited individuals participated in the intervention. The mean number of watching modules was 2.8 ± 0.8 for breast cancer patients and 2.6 ± 0.8 for caregivers. 52.2% of patients and 47.8% of caregivers watched three modules in the intervention. For the loving-kindness meditation practice, 34.8% of patients practiced for 15 to 21 days and 52.2% of caregivers practiced for &lt; 7 days. For the three good things practice, 56.5% of patients and 47.8% of caregivers practiced for 7 to 14 days. Mean scores on the acceptable scale for patients and caregivers were 27.61 ± 3.71 and 25.91 ± 3.01, respectively. The results of a linear mixed model showed that the preliminary effects of the intervention on participants’ resilience, hope, perceived benefits, positive aspects of caregiving, and quality of life were significant (<i>p</i> &lt; 0.05).</p> Conclusions <p>The online positive psychological interventions for breast cancer patients and family caregivers are feasible, and has significant preliminary effects on participants’ resilience, hope, perceived benefits, positive aspects of caregiving, and quality of life, however, the effects warrant further randomized controlled trials.</p> Trial registration <p>Chinese Clinical Trial Registry (ChiCTR) ChiCTR2300072809. Registered on 26 June 2023Retrospectively registered.</p>