The 19-year-old signed his first professional contract with the Royals last summer having come through the Academy and could make his league debut on the English Riviera.
He has represented England at youth level and recently had a spell on loan with Blue Square Bet South side Basingstoke.
The Gulls were in the market for a defender after losing centre-half Chris Robertson to Preston on transfer deadline day.
He is eligible for the Gulls' home clash with Shrewsbury Town this weekend and can remain with them until the end of the play-offs should they make it that far for the second straight season.
"He will train with us tomorrow (Thursday) and finalise his paperwork," Ling told the club's official website.
"The deal is actually a youth loan because Angus is 19 and that enables us to keep him here for longer, if required.
"He is a big centre-half and rated very highly by the people at Reading. They see him as a massive prospect for the future and are happy for him to continue his education with us.
"It gives us extra cover at centre-back after the departure of Chris Robertson last week."
