Well, according to CNR or China National Radio, the accident was because of failure of J-15's flight control system.
CNR said that during the landing training, Pilot Zhang Chao's J-15 suddenly had an elec flight control system failure, he tired his best to control the plane by pushing rod, however it did not work, so he ejected and hit the ground.
It seems that the J-15 had the incident when it is going to land, so its altitude is pretty low, the pilot tried to push the rod, so maybe J-15's nose was rising up quickly, when he ejected, the direction of ejection could be level, or even towards to ground. Thus there was no time for him to adjust, he directly hit the ground after ejection.
Unfortunately, PLAN lost a great and experienced pilot. T-T