The first look picture shows Kristen dressed in a lavish red coat, dyed blonde hair and a black veiled hat.
Directed by Pablo Larrain (Jackie) and reportedly written by Peaky Blinders creator Steven Knight, it will be set in Sandringham, and will show Diana as she realises her marriage is on the rocks.
The drama takes place over three days in one of Princess Diana's final Christmas holidays in the House of Windsor in their Sandringham estate, according to Deadline.
The producers have not confirmed who will play Prince Charles and their two children - Prince William and Prince Harry. However, Timothy Spall, Sally Hawkins and Sean Harris have all been cast in unspecified roles.
The film is currently shooting in Germany, with further scenes to be filmed in UK soon.
Kristen told Deadline: "Spencer is a dive inside an emotional imagining of who Diana was at a pivotal turning point in her life. It is a physical assertion of the sum of her parts, which starts with her given name; Spencer. It is a harrowing effort for her to return to herself, as Diana strives to hold onto what the name Spencer means to her.'
When Kristen was cast as Princess Diana in June last year, Director Pablo Larrain told Deadline: "We decided to get into a story about identity, and around how a woman decides somehow, not to be the queen.
"She's a woman who, in the journey of the movie, decides and realises that she wants to be the woman she was before she met Charles."
Princess Diana and Prince Charles wed in 1981, and their marriage lasted eleven years, publicly breaking down in 1992.
The pair divorced in 1996, and Diana tragically died in a car crash a year later, aged just 36.
Spencer is due to be released in 2021 though a specific date has not yet been announced.