Dilated Cardiomyopathy (DCM) and cardiac conduction disease (CCD) are two kinds if diseases that can induce heart failure, syncope and even sudden cardiac death (SCD). DCM patients can experience CCD at the same time. In recent research, some disease-causing genes and variants have been identified in patients with DCM and CCD, such as Alpha-Actinin-2 and TNNI3 Interacting Kinase (TNNI3K). In this study, we employed whole-exome sequencing (WES) to explore the potential causative genes in a Chinese family with DCM and CCD. A novel splice site mutation (c.333 + 2 T > C) of TNNI3K was identified and co-segregated with the affected family members. This novel mutation was also absent in 200 healthy local controls and predicted to be disease-causing by Mutationtaster. The splice site mutation (c.333 + 2 T > C) may result in a premature stop codon in exon 4 of the TNNI3K gene and can induce nonsense-mediated mRNA decay. Real-time qPCR also confirmed that the level of TNNI3K mRNA expression was decreased significantly compared with the controls, which may lead to myocardial structural disorder and arrhythmia. In this study we reported the third novel mutation of TNNI3K in DCM and CCD patients which further supported the important role of TNNI3K in heart development and expanded the spectrum of TNNI3K mutations. The results may contribute to the genetic diagnosis and counseling of families with DCM and CCD.