Offensive speech has a serious negative impact on social stability. Currently, automatic detection of offensive speech focuses on a few high?resource languages, and the lack of sufficient offensive speech tagged corpus for low?resource languages makes it difficult to detect offensive speech in low?resource languages. In order to solve the above problem, a cross?language unsupervised offensiveness transfer detection method was proposed. Firstly, an original model was obtained by using the multilingual BERT (multilingual Bidirectional Encoder Representation from Transformers, mBERT) model to learn the offensive features on the high?resource English dataset. Then, by analyzing the language similarity between English and Danish, Arabic, Turkish, Greek, the obtained original model was transferred to the above four low?resource languages to achieve automatic detection of offensive speech on low?resource languages. Experimental results show that compared with the four methods of BERT, Linear Regression (LR), Support Vector Machine (SVM) and Multi?Layer Perceptron (MLP), the proposed method increases both the accuracy and F1 score of detecting offensive speech of languages such as Danish, Arabic, Turkish, and Greek by nearly 2 percentage points, which are close to those of the current supervised detection, showing that the combination of cross?language model transfer learning and transfer detection can achieve unsupervised offensiveness detection of low?resource languages.