The invention provides an online examination method and system based on a cloud platform. The online examination method based on the cloud platform comprises the following steps: providing a plurality of pieces question information, wherein each piece of question information comprises a question and one correct answer and N wrong answers corresponding to the question; receiving an examination instruction input by a user; selecting one piece of target question information; and generating a test choice question, wherein the test choice question comprises a target question and M+1 target alternative options, the M+1 target alternative options being formed by one target correct answer and M target wrong answers, the M target wrong answers being selected randomly from N target wrong answers. The method and system solve the problem that an existing examination system cannot adjust the question answers, expand functions of the online examination system and prevent the problem of uncertain examination results due to answer cramming of students.