Cryptosporidium parvum is a major zoonotic pathogen responsible for outbreaks of severe diarrhea in humans and calves. Almost all investigations of cryptosporidiosis outbreaks caused by C. parvum have focused on its IIa subtype family in industrialized nations. From December 2018 to April 2019, approximately 200 neonatal calves on a large cattle farm in Hebei Province, China presented watery diarrhea and over 40 died. To investigate the cause of the outbreak, 179 and 223 fecal specimens were collected during and after the diarrhea outbreak from the farm, including 40 and 56 from neonatal calves, respectively. Among them, 18 fecal specimens from ill calves during the peak of the outbreak were analyzed for four common enteric pathogens using enzymatic immunoassay (EIA), 75 additional specimens from neonatal calves were tested for rotavirus by EIA, and all specimens were analyzed for Cryptosporidium spp. using PCR and sequencing techniques. Of the initial 18 specimens from sick calves, ten were positive for C. parvum, five for rotavirus, and one for coronavirus. The overall prevalence of rotavirus in neonatal calves was 20.0% (15/75), with no significant differences during (21.6% or 8/37) and after (18.4% or 7/38) the outbreak. In contrast, the prevalence of C. parvum was significantly higher during the outbreak (60.0%, 24/40) than after the outbreak (30.4%, 17/56). C. parvum infection was associated with the occurrence of watery diarrhea in neonatal calves (odds ratio = 11.19), while no association was observed between C. bovis infection and diarrhea. All C. parvum isolates were identified as subtype IIdA20G1. Older animals were infected with C. bovis, C. ryanae, C. occultus, and C. andersoni. This is one of the few reports of outbreaks of severe diarrhea caused by C. parvum IId subtypes in calves. More attention should be directed toward preventing the dissemination of C. parvum in China.