Taking application of conservation theorem of momentum for example, the approaches to cultivating students′ ability of thinking are explored in teaching of Mechanics. Ability of thinking can be cultivated in many ways: teachers' asking questions can stimulate students′ active thinking, students' questioning can help them to form autonomic thinking habit, teachers' training the students' scientific logic thinking can cultivate the students' thinking ability. And students′ ability of thinking is also cultivated by many approaches: scientists′ thinking process could inspire students′ thinking consciousness, combination of experiment with practice in mechanics teaching may broaden students′ thinking, deducing and training principium and formula can help to form rigorous thinking mode, adoption of divergent thinking and reversed thinking helps to exploit students′ innovative thinking and creative thinking.