Chang began his law enforcement career as a detective within the National Police Agency's Criminal Investigation Bureau.

[1] As police bureau chief at Chiang Kai-shek International Airport, Chang worked to stop drug smuggling and took part in the care of political asylum seekers.

[2][3] In October 2002, Chang and the National Immigration Agency's Tseng Wen-chang were questioned by Control Yuan members Lee Shen-yi and Lin Shih-chi regarding the defection of Republic of China Army Lieutenant Wang Yi-hung.

[1][5] That same month, Chang participated in a drug raid of the largest nightclub in Taiwan, alongside interior minister Yu Cheng-hsien and Criminal Investigation Bureau commissioner Hou You-yi.

[19][20] Mayor of Taipei Ma Ying-jeou criticized the interference of Chang and the National Police Agency, stating "I think it was illegal, unfeasible and unnecessary for the NPA to intervene in the dispersion" of demonstrators protesting the presidential election results along Ketagalan Boulevard.