Objective This study aimed to analyze the clinical manifestations, coronary artery lesion (CAL) features, and independent risk factors/predictive indicators for intravenous immunoglobulin (IVIg) non-response in infants with Kawasaki disease (KD) under 6 months of age. Methods A retrospective analysis was performed on clinical data of 435 KD infants aged 0-12 months who were hospitalized January 2020 to December 2024. Patients were stratified into the 0-5 months group (under 6 months) and 6-12 months group; the 0-5 months group was further subdivided into incomplete KD (iKD) and complete KD (cKD) subgroups. Differences in core clinical characteristics between the 0-5 months and 6-12 months groups were analyzed, and phenotypic variations between the iKD and cKD subgroups were compared. Multivariate logistic regression was used to identify risk factors for CAL and IVIg non-response in the 0-5 months group. Receiver operating characteristic (ROC) curves were constructed to evaluate the predictive efficacy of candidate variables for IVIg non-response. Results A total of 179 patients (41.1%) were included in the 0-5 month group. Compared with the 6-12 months group, the 0-5 months group exhibited a significantly higher incidence of Bacille Calmette-Guérin (BCG) scar erythema/swelling and elevated inflammation-related markers such as platelet count, alongside lower frequencies of typical KD symptoms like conjunctival hyperemia and reduced albumin/hemoglobin levels. The incidence of CAL was also higher in the 0-5 months group (47.5%; all P<0.05). Within the 0-5 months group, 73 patients (40.8%) were diagnosed with iKD, presenting fewer typical symptoms, a higher CAL incidence (58.9%), and a higher IVIg non-response rate than the cKD subgroup (all P < 0.05). Multivariate regression revealed that BCG scar erythema/swelling was independently associated with an increased risk of CAL (OR=2.81, 95% CI: 1.44-5.48, P=0.002). Conjunctival hyperemia was inversely correlated with IVIg non-response (OR=0.15, 95% CI: 0.03-0.89, P=0.036), while a neutrophil-to-lymphocyte ratio (NLR)≥3.78 (OR=3.17, 95% CI: 1.37-7.31, P=0.007) and fever duration≥6.5 days (OR=1.38, 95% CI: 1.03-1.84, P=0.029) were identified as independent risk factors for IVIg non-response. The combined model of NLR≥3.78 and fever duration≥6.5 days yielded an area under the ROC curve (AUC) of 0.888 for predicting IVIg non-response, with a sensitivity of 0.867 and specificity of 0.841. Conclusions KD infants under 6 months of age often present with atypical manifestations. iKD in this age group is associated with a higher incidence of CAL and IVIg non-response. BCG scar erythema/swelling is closely linked to CAL development. The combination of NLR≥3.78 and fever duration≥6.5 days has robust predictive value for IVIg non-response, while conjunctival hyperemia may facilitate the early identification and management of IVIg-nonresponsive KD in infants under 6 months.