The State-owned Assets Supervision and Administration Commission (SASAC) has issued regulations on the part-time job and salary of leaders of state-owned enterprises: leaders of enterprises who are approved to take part-time jobs in subordinate enterprises or other enterprises shall not receive any part-time salary, nor shall they reimburse any expenses unrelated to part-time enterprises. The paid part-time remuneration and the reimbursed expenses unrelated to the enterprise must be turned over to the financial department of the enterprise in full, and shall not be taken as existing.
Therefore, at least for state-owned enterprises, it is necessary to correct the illegal behavior of senior executives of parent companies taking part-time jobs in subsidiaries.
As for whether it is illegal or not, it mainly depends on whether related party transactions have caused the loss of state-owned assets and whether some people have pocketed their own money and harmed public and private interests.