Orientalists have been researching and expressing their views on Mahdism and related issues for more than 150 years. These researches have continued to the present time which have been accompanied by ups and downs and specific features of each period over time. The following research- carried out with a descriptive-analytic method, on the basis of investigation and analysis of views of over 200 Orientalists in the last two centuries, organized in an academic dissertation, seeks to identify and introduce the most important drawbacks and harms of Orientalists’ researches in the field of Mahdism, especially in the contemporary period. Based on the results of this study, the main defects and harms of active and knowledgeable Orientalists on the subject of Mahdism are as follows: one-sided or presuppositional view, incorrect foundation of the intellectual system, incorrect use of Islamic and Shiite sources, adoption of inappropriate approaches and methods with the subject, and finally, heterogeneous generalization or partiality and without prioritization in different areas of principles, sources and approaches or selection of subjects.