In 1799 he became involved in the compilation of the Dai Nihon-shi (Great History of Japan) being undertaken by the Mito school.
In 1825 he wrote his Shinron ("New Theses"), a collection of essays that dealt with issues such as Tokugawa defence policy and the perceived threat that Western ships presented to Japan. Aizawa also tried to describe conditions in the West and theorize why Western states had become so powerful; in his opinion Westerners used religion to enforce conformity of the masses. He believed that Japan would need to take up its own state religion and discussed the concept of kokutai ("national polity"), in this context. The Shinron would become an important work for the sonnō jōi movement and his theory of the Kokutai would be developed by future thinkers.
In 1840 Aizawa became the first head of professors of the Mito school's Kōdōkan but was forced to resign in 1844 when Tokugawa Nariaki resigned as domain leader. He later returned to the Kōdōkan.