Breast cancer is the most common cancer among women worldwide. Adjuvant chemotherapy has significantly reduced mortality but increased cognitive impairments, including attention function, making quality of life issues a crucial concern. This study enrolled nineteen breast cancer patients who were treated with standard chemotherapy within 6 months and 20 sex-matched healthy controls to investigate the brain effects of chemotherapy. All participants underwent resting-state functional magnetic resonance imaging (rs-fMRI) with mean fractional amplitude of low-frequency fluctuation (mfALFF) analysis and were correlated with neuropsychological tests, including the Mini-Mental State Examination (MMSE), the Cognitive and Affective Mindfulness Scale-Revised (CAMS-R), and the Impact of Event Scale-Revised (IES-R), to explore the possible underlying mechanism of cognitive alternations. We found increased mfALFF over the frontoparietal lobe and decreased mfALFF over the occipital lobe in the cancer patients compared with the healthy controls; the altered brain regions may be associated with the dorsal attention network (DAN) and may be explained by a compensatory mechanism. Both MMSE and CAMS-R scores showed a positive correlation with mfALFF in the occipital lobe but a negative correlation in the frontoparietal lobe. By contrast, IES-R scores showed a positive correlation with mfALFF in the frontoparietal lobe but a negative correlation in the occipital lobe. These alterations are potentially related to the effects of both chemotherapy and psychological distress. Future research involving a larger sample size of patients with breast cancer is recommended.