Aconitum leucostomum Worosch is a traditional Chinese medicine (TCM) and has a broad spectrum of health effects, but with a narrow therapeutic window. It is important to identify both the therapeutic ingredients and the toxic components to better utilize this TCM. The present study investigated the cardiotoxicity of the selected compounds in Aconitum leucostomum Worosch. The effects of extract of A. leucostomum Worosch and the isolated compounds on cardiocardiomyocytes were evaluated in vitro. Five known compounds in this TCM, including three C18-diterpene alkaloids, lappaconitine (2), N-deacetyllappaconitine (3), and ranaconitine (5), and two C19-diterpene alkaloids, delvestidine (1) and anthranoyllycoctonine (4), were isolated from A. leucostomum Worosch. The cardiotoxicity of these components and extract fractions, as measured by lactate dehydrogenase release and apoptosis, was ranked as follows, in descending order: delvestidine > anthranoyllycoctonine > pH 4 fraction > pH 8 fraction > aconitine > N-deacetyllappaconitine > ranaconitine > lappaconitine. The cytotoxicity of these compounds was shown to be dose-dependent, with delvestidine (1) and anthranoyllycoctonine (4) being the two most toxic compounds to cardiomyocytes in our assays. These results provide a basis for future rational use of this TCM, reducing side effects while retaining therapeutic effects.