With the consent of the case-handling organ, you can write a letter in the detention center. According to Article 34 of the Measures for the Implementation of the Regulations on Detention Centers in People's Republic of China (PRC), communication between criminals and their close relatives living in China must be approved by the case-handling organ. If a meeting is required, it must be approved by the competent bureau or director of the public security organ at or above the county level.
A detention center is an organ that holds criminal suspects who have been arrested and detained according to law. If the remaining sentence of a criminal sentenced to fixed-term imprisonment is less than three months before delivery for execution, the detention center shall execute it on his behalf.