In the 21st century, cultivating complex skills has become an urgent educational need, especially in vocational training and learning. The widespread lack of formative assessment during complex problem-solving and skill-learning processes limits students’ self-perception of their weakness and teachers’ effective monitoring of students’ mastery of complex skills in class. To investigate methods of how to design and carry out formative assessments for the learning of complex skills, a whole learning process-oriented formative assessment framework for complex skills was established. To verify the feasibility and effects of the formative assessment, a controlled experiment involving 35 students majoring in industrial robotics from one of Shanghai’s Technical Institutes was designed. The results indicate that the formative assessment can effectively promote students’ learning of conceptual knowledge and the construction and automation of cognitive schema as well as improve students’ competency in the implementation and transference of complex skills. In addition, the formative assessment, which can optimize the allocation of psychological effort by increasing the proportion of germane cognitive load within the overall cognitive load, does not place an additional cognitive load on students. It can provide methodological support for promoting students’ learning of complex skills.