The Avon and Somerset force has conceded Dale McAlpine did nothing wrong and he should not have been told to move on.
Sergeant Jonathan Raisey said: "Instead of dispersing your group, I would have expected that my officers would have remained at the location to protect the right of your client's group to free speech."
Mr McAlpine and a group of other evangelists were telling people in central Bath about Jesus Christ on 25th June 2018 when they encountered angry members of the public.
Arriving on the scene in two police cars with blue lights flashing, officers issued a dispersal notice which forced the group to leave the area. One claimed the preachers were committing a "hate crime".
With the support of the Christian Institute campaign group, Mr McAlpine mounted a complaint which has now prompted the apology.
Director of the Institute, Colin Hart said: "We don't want dispersal notices to be similarly misused. Vigilance is needed.
"Avon and Somerset Police's response to the complaint underlines the freedom of Christians to preach about Jesus on our streets."
The force says all police staff have been contacted "to ensure that they understand the importance of freedom of expression".
Colin Hart from the Christian Institute added: "Christians have the freedom to preach on the streets.
"That right was hard won down the centuries and it is important that it is upheld.
"The treatment of the street preachers was very concerning.
"The law cannot be used to clamp down on legitimate free speech."
Having examined the actions of the street preachers - by watching police body camera footage - Sergeant Raisey concluded their behaviour was "absolutely acceptable" and "officers should have acted differently."
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