An evening rush hour scene in Sihui station, on June 22, 2016.[Photo by Zhang Xinghai/For China Daily]
With a young son to care for, he had few opportunities to go on photo shoots, so snapping the daily subway commute was an easy approach.
He seldom talks to his subjects. Some have responded with a smile; some have looked at the ground or stared back angrily; while a few have demanded he delete their photos at once.
Aiming his lens at strangers was difficult. At first, he only dared to capture people's backs or beggars.
Once, he tried to capture a romantic moment when he noticed a girl closing her eyes as she accepted a kiss from her foreign boyfriend.
He summoned the courage to approach the girl and raised his camera, but the moment he pressed the shutter, she suddenly opened her eyes in anger and suspicion. She and her boyfriend wanted to report him to the police, so Zhang exposed the whole film to placate them.
He has tried asking permission before taking photos, but he found the subjects became less natural and more self-conscious. Now he openly wears his camera around his neck to make it clear he's taking photos.
"You can't feel embarrassed about taking photos," he said.
He has a new digital camera and refuses to use a mobile phone as this would appear "stealthy". He had planned a book, but publishers told him an unknown photographer's work would not sell. However, since he began sharing his photos on social media in April, they have drawn millions of hits.
His work is also earning critical acclaim for its simple depiction of people's lives.
"I believe what he sees in these faces is the photographer himself - lonely, curious, depressed, but appreciating city life. Different images repeat the same feeling, which is the power of these snapshots," said Yan Zhigang, an art critic.
Zhang's photos also record changing lifestyles in China. In 2006, the subway was packed with newspaper readers, but now people stare at their phones or laptops; passengers used to wear simple masks to avoid germs, but now they wear more sophisticated masks to keep out the smog; more people appear with tattoos or dressed in punk regalia, reflecting a more diverse urban culture.
Zhang now plans to leave the subway to return to his hometown and photograph the country life he misses.