Court of Appeal judges sitting at Cardiff Crown Court said they would not grant leave for appeal.
One of Watkins's two female accomplices also had her appeal bid turned down.
Although 37-year-old Watkins did not challenge his 13 sex offence convictions, the disgraced rock star complained he was treated too harshly by sentencing judge Mr Justice Royce.
Watkins was handed a 29-year jail term as well as an additional six years on licence. Among the offences he pleaded guilty to were the attempted rape of an 11-month old child as well as encouraging a fan to abuse her baby via webcam.
Police also found a stash of indecent videos of children - some of which he had made himself - at his home.
Mr Justice Royce described Watkins as a dangerous sexual predator, as he sentenced the singer last December, adding: "Young females need protection from you."
Defending Watkins at yesterday's hearing was QC Sally O'Neil. She said her client was not given enough discount for pleading guilty.
She said: "By pleading guilty my client averted the need for a most distressing and unpleasant trial for all those concerned."