Similar love triangle situation, historical setting, political turmoil, dramatic melancholic love stories.
There's a suspicion among the imperial family to run for the throne. Even though the male lead of both series already take an oath to stay loyal, the suspicion is still exist. Male lead of both dramas are great general who has been spending their life for peace even though suspicion of them beings rebels exists. Both of the drama leads also can't be together in the end because of their priorities and responsibilities.
In the Legend of the Phoenix, the ML and the FL are so in love with each other. When the ML goes to war, the FL promises that she would wait for him to return. But the ML is in grave danger in losing his life unless the FL marries the Emperor. So she marries the Emperor to save the ML. When ML returns from war, he finds his love married. He's in immense sorrow.
Both dramas are based on the same history surrounding Hong Taiji and Da Yu'er. The characters in The Promise of Chang'an are counterparts of historical figures in Qing Dynasty during the respectful time period:
He Lan Mingyu - Da Yu'er
Xiao Chengxu - Dorgon
Xiao Chengru - Hong Taiji
He Lan Wanyin - Ha Lan Zhu
The only significant difference is the Promise of Chang'an is set in a fictitious dynasty rather than the Qing Dynasty due to censorship.
It's also ironic that Han Dong (who played Dorgon in 'In love with Power') plays Xiao Chengrui, Hong Taiji's counterpart.
He Lan Mingyu - Da Yu'er
Xiao Chengxu - Dorgon
Xiao Chengru - Hong Taiji
He Lan Wanyin - Ha Lan Zhu
The only significant difference is the Promise of Chang'an is set in a fictitious dynasty rather than the Qing Dynasty due to censorship.
It's also ironic that Han Dong (who played Dorgon in 'In love with Power') plays Xiao Chengrui, Hong Taiji's counterpart.