A PhD dissertation written by President Xi Jinping 13 years ago, in which he urged reform of the hukou, or household registration system, was made public on Wednesday.

The system has created a large income gap between rural and urban residents that should be corrected in a prudent and proactive manner, Xi wrote.

The dissertation, part of a library collection at Tsinghua University, was written in December 2001 when Xi, then governor of East China's Fujian province, was studying economics at Tsinghua for a doctoral degree.

In the dissertation, Xi said that most migrant workers could not enjoy many social services because of restrictions inherent in their hukou status.

Under current rules, people with urban hukou enjoy better social services including health, housing, education and pensions. Even though many rural people have migrated and lived in a city for years, they are barred from many social services because of their lack of urban hukou.

The historical trend points to the abolition of the hukou system, which is based on birth, Xi wrote.

The government should face reality in a reasonable and brave manner, and reform the hukou system to level out social services, he wrote.

The dissertation also mapped out specific steps for hukou reform. Small towns and cities could take the first step to lift restrictions, it said, while medium and large cities could follow.

In the dissertation, Xi suggested that migrant workers who have lived in a city for years and have stable incomes could be given priority in their applications for urban hukou.

Official statistics show that China had 166 million migrant workers in cities and towns away from their documented homes at the end of last year. The figure increased to 174 million by the end of June this year.

Xi expressed his concerns about the problems of hukou many times before he was elected president.

In July 2001, when he was governor of Fujian, Xi told a local magazine that the government would promote the process of urbanization and make reforms to hukou, farmland and related matters.

During the annual session of the National People's Congress in March 2013, Xi asked Zhu Xueqin, a migrant worker, whether she had obtained hukou in Shanghai municipality, the Beijing News reported.

According to the report, Zhu replied that she got Shanghai hukou only after being designated one of China's 1,000 excellent migrant workers in 2008 by the State Council, China's cabinet.